Seasonal patterns in diagnosis were examined for 2403 melanomas of the skin and 238 melanomas of the eye diagnosed among residents of upstate New York State in 1975 to 1979 and reported to the population-based New York State Cancer Registry. The previously reported summer peak in diagnosis of melanoma of the lower extremity in females was confirmed and shown to hold for melanomas diagnosed at both early and late clinical stages; increased recognition of earlier stage melanomas in summer months is an unlikely explanation for this peak. For skin melanomas at other anatomic sites, seasonal patterns by sex and clinical stage at diagnosis were more complex. A previously unreported seasonal pattern was evident for trunk melanomas in males, with a March-April peak. Significant seasonal variations in diagnosis of melanomas of the eye were evident for both males and females, but the seasonal patterns differed between the sexes. Possible explanations for these findings on skin and eye melanomas were discussed with reference to psychosocial and hormonal factors as well as ultraviolet stimulation.