Etiology of malignant melanoma in persons younger than 20 years of age was examined using data from two sources: medical records of 78 patients with this diagnosis at six hospitals, and information on 101 individuals included in 10 U.S. population-based SEER cancer registry areas between 1973-1976. Annual melanoma incidence rate was 3.4 per million in boys, 3.9 per million in girls, and 10-fold greater in white children than in black children. Melanoma was seven times more frequent in the second decade of life than the first. Skin was the primary site of melanoma in approximately 90% of the children in the two study series. The predominant cutaneous sites were head, neck, and trunk among boys, and arms and legs among girls. These variations by age, race, and sex suggest the etiologic role of cumulative skin exposure to sunlight, particularly in two patients with xeroderma pigmentosum. In 14 patients in the hospital series, melanoma was reported to develop within pigmented nevi that were present at birth.