One hundred ten skin biopsies were taken from 55 consecutive autopsies to evaluate the number and location of mast cells in the dermis. For each autopsy, one biopsy was taken from the V of the neck (sun-exposed area) and the other from the upper thigh (non-sun-exposed area). Normal-appearing skin was biopsied. There were 28 men and 27 women ranging in age from 16 to 94 years. Fifty-three patients were Caucasian and two were Negro. Mast cells were counted in 10 random high-power fields in the papillary dermis only. The average number of mast cells per high-power field (X 400) in sun-exposed skin for both men and women was 8.19 (65/mm2 or 13,000/mm3) with one standard deviation of 4.08, while that of non-sun-exposed skin was 7.52 (60/mm2 or 11,900/mm3) with one standard deviation of 3.62. The difference between the number of mast cells in sun-exposed and non-sun-exposed skin was not statistically significant. In addition, no statistically significant differences were observed for the average number of mast cells per high-power field in regard to sex or the presence of malignancies.