A 3 year survey of practicing veterinarians in the State of Wisconsin revealed a strong seasonal variation (peak incidence in January and February) for spontaneous canine diabetes mellitus (SCDM) in pet dogs. This seasonal incidence was quite similar to that observed in insulin-dependent diabetes mellitus (IDDM) in humans. SCDM is etiologically heterogeneous, it has dissimilar etiologies as compared to IDDM, and it is unlikely that viral infections play a causal role. For these reasons, it is concluded that the finding of a similar seasonal incidence in SCDM and IDDM suggests that the seasonal incidence of IDDM most likely represents the effects of, as yet undefined, stresses which precipitate the diabetic state in humans and dogs with pre-existent islet dysfunction of varying causes.