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Age-standardized mortality rates from malignant melanoma and from other skin tumours, for each Canadian province and United States state, over the period 1950–67 were examined. Both rates show a strong negative relationship with the geographical latitude of the largest city in the province or state. The rates of change of mortality rate with latitude in the two disease categories are similar in each sex, being greater in males than in females. Longitude, altitude, and mean income are not related to the skin cancer mortality rates. Estimates of the annual ultraviolet radiation in the erythema-producing wavelengths have been made for each state or province. These show a strong negative association with latitude, and a similar degree of correlation with skin cancer mortality rates as does latitude. From this and previous work we suggest that ultraviolet radiation is involved in the aetiology of melanoma as well as of …