A 6-month-old male crossbred dog weighing 0.78 kg was presented with acute bilateral immature cataracts, intermittent diarrhea and growth retardation. The clinical manifestations and laboratory findings were suggestive of concurrent juvenile diabetes mellitus (DM) and exocrine pancreatic insufficiency (EPI). Moreover, the DM was associated with a decreased level of serum insulin-like growth factor I. Histological examination revealed a markedly lower number of pancreatic islets and acinar cells. This case shows that juvenile-onset DM can occur simultaneously with EPI and result in growth retardation, acute cataract formation and a high cortisol concentration.