This study investigated the risk factors for diabetes mellitus in dogs reviewing medical records of 6860 dogs with diabetes mellitus (DM) and 6070 control dogs with no DM from veterinary teaching hospitals between 1970 and 1999. The prevalence of DM increased from 19 cases per 10000 admissions per year in 1970 to 64 in 1999. However, mortality decreased from 37% to 5%. The highest prevalence was in dogs aged 10-15, and dogs with a weight less than 22.7 kg and female dogs had greater risk compared with heavier dogs and males respectively.