A retrospective analysis of patient data from the Clinic for Equine Internal Medicine (PFI) at the Justus Liebig University (JLU) Giessen/Germany formed the basis for the main objective of this thesis, the development of a species-specific disease classification for internal medicine diagnoses in horses. The data included all horses, ponies and donkeys that were presented to the PFI from 2000 to 2020. The main parameter was the individual principal diagnosis, which was classified and coded using an initial classification of diagnoses. This first diagnosis code contained 14 main groups, mostly based on organ systems. Each main group was divided into a maximum of four levels in order to hierarchize the diagnoses. For each case, one principal and up to two secondary diagnoses, outcomes and patient-related information such as age, gender, race, length of stay, duration of illness and any re-presentations were transferred to an Excel spreadsheet. The data of 14,034 PFI patients were recorded over 21 years (January 2000 to December 2020). 484 companion animals were excluded, resulting in 13,550 main diagnoses. The annual number of cases varied between 497 (2000) and 822 (2019), with a median of 626. The average age of patients increased from 9.9 (2000) to 13.7 years (2020), which resulted in a highly significant increase (p < 0.0001) of approx. 2 months per year. The gender distribution showed slight increases in geldings and mares, while the already low number of stallions continued to decrease. Warmbloods formed the largest breed group with 56.3%. Over the years, their percentage share decreased, while ponies increased (p < 0.0001). The length of inpatient stay decreased significantly (p < 0.01) from a median of 5.0 (2000-2002) to 3.2 days (2018-2020). The vast majority (83.5 %) of patients presented once, 11.9 % had at least one re-presentation due to the same condition and 4.6 % were re-presented due to other symptoms. Most patients presented with a peracute event, the majority of whom were colics. 67.4 % of all patients could be discharged, 18.6 % were referred to the Clinic for Equine Surgery and Orthopaedics (PFC) for colic surgery (91 %). In the PFI, 13.2 % of patients had to be euthanized, 0.2 % died and 0.3 % were referred to external clinics. Referrals to the PFC decreased over time, while the proportion of euthanized and deceased patients increased, which could be related to the older age of the patients. Diseases of the digestive tract predominated with 9,039 cases (66.7 %). These were mainly represented by colic. Gastric diseases increased significantly from 11 cases in the first three years to 193 cases in the last three years. Respiratory diseases were the second largest group with 1,558 cases (11.5 %) and chronic obstructive bronchitis (equine asthma) was the most common diagnosis within respiratory diseases (61.9 %). The cardiovascular system were affected in 795 cases (5.9 %), with 400 of the 634 cardiac cases proving to be heart valve insufficiencies. Systemically affected horses made up the fourth largest group with 4.0 % (n = 544) of all patients; patients with fever (39.3 %) and emaciation (16.7 %) of unknown origin were most frequently classified here. Diseases of the skin, hair and lymph nodes accounted for 2.2% of the total number of cases (302 cases). Due to inclusion of infections of the lymph nodes with Streptococcus equi. ssp. equi (strangles) in this group, these cases included accounted for 22.2 % in this group. Urinary and reproductive organs were affected in 242 cases (1.8 %), with a noticeable increase in renal insufficiency from 5 patients in the first three years to 24 cases in the last three years. Between 58 and 229 cases were categorized in the other seven main groups. On the basis of these evaluations and inspired by the WHO's international statistical Classification of Diseases and Related Health Problems, the first diagnosis code was optimized. Main groups such as skin/hair and lymph nodes, cardiovascular system and urinary and reproductive system were divided, new main groups for lymphatic and haematopoetic diseases, sensory organs and nutritional disorders were created and numerous diagnoses were added. Poisoning, behavioral disorders and adverse drug reactions can also be classified. The use of the optimized diagnosis classification in equine practices and clinics could make a significant contribution to gaining precise knowledge of diagnosis frequencies in equine medicine based on high case numbers in order to derive research and training priorities. Translated with DeepL.com (free version) and adapted by the author.