A detailed histopathologic study was performed on 128 nevi removed from 86 pregnant white patients. None of the patients had clinical evidence of the dysplastic nevus syndrome, and none of the excised nevi were clinically suspicious for melanoma. One third of patients reported some change in nevi during pregnancy. Nevi from an age-matched male control population (50 patients) had a spectrum of histologic features essentially identical to the pregnant patients, while nevi from a female control population (51 patients) showed slightly less atypia according to our criteria. The differences found between the nevi from pregnant and control women suggest that a mild degree of histopathologic atypia or "activation" is associated with pregnancy. However, in the population studied, these changes were never of sufficient degree to result in diagnostic confusion.