Biopsy specimens were taken from the pigmented lesion in 13 patients with nevus spilus and in 13 patients with Becker’s nevus. Control specimens were taken from the apparently normal skin either adjacent to the lesion or on areas elsewhere in 11 patients out of the 26 patients. The specimens were fixed in formal saline, cut at seven micra in thickness, and sections were stained with toluidine blue. Measurements were made on four sections of the dermis in each specimen, avoiding the follicular areas deliberately. The counting of mast cells in each section was made on a 2,000
μ width of dermis and on a 500
μ depth of dermis just below the epidermis. The average value was described, thus, as the number of mast cells per square mm of the dermis. The logarithmic expression of the number of mast cells was found to approximate the normal distribution in eash group. Changing the logarithmic expression, the sample mean was 34.8/mm
2 in the control, 100.4/mm
2 in nevus spilus, and 71.7/mm
2 in Becker’s nevus. The unbiased estimate of population variance ranged from 0.0202 to 0.0282. The number of mast cells was estimated to increase significantly in patients with nevus spilus and with Becker’s nevus (
α=0.01). The historical background of the two diseases was also discussed.
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