The effects of pregnancy on the pathophysiology of melanoma remain unclear. Although a gender-specific advantage for women vs. men is seen for characteristics such as stage at presentation, site of primary tumour, and survival time, an adverse effect of pregnancy on melanoma development and progression has been reported. In a retrospective study, we investigated the tumour characteristics of women who developed pregnancy-associated melanoma, and compared them with melanomas arising in non-pregnant women of child-bearing age. The patient records of the Massachusetts General Hospital Pigmented Lesion Clinic were reviewed, and 465 women of reproductive age (16-45 years) who developed melanoma were identified. Of these, in 45 women (age 21-42 years) there was a close temporal relationship between diagnosis of the tumour and pregnancy. Clinical and histological characteristics of the primary tumours were recorded. Differences in tumour thickness, site and histological type were analysed. The mean thickness of pregnancy-associated melanomas was significantly greater than that of non-pregnancy-associated tumours (2.28 vs. 1.22 mm, respectively; P < 0.007). No differences in histological type (P = 0.64) or site (P = 0.74) of the primary tumours were found between the two patient groups. Not surprisingly, multivariate analysis revealed that tumour thickness was a statistically significant variable in determining prognosis (P = 0.001). An unexpected finding, on multivariate analysis, was a possible pregnancy-associated prognostic advantage (P = 0.08). Melanomas arising during pregnancy are thicker, but are not necessarily associated with a less favourable prognosis than tumours arising in non-pregnant women of child-bearing age.(ABSTRACT TRUNCATED AT 250 WORDS)