Image cytometry was used to assess seasonal variation in the proliferation fraction of Australian common nevi. Twenty pairs of nevi were evaluated. One member of each pair had been excised during the Australian summer, and the other member of the pair had been excised during the winter. The nevi were matched for age, sex, and body site. From each nevus, 6-microns sections were Feulgen-stained and evaluated with a CAS 200 image analyzer, which was used to obtain DNA histograms. Proliferation fractions were calculated from the histograms. The proliferation fraction of the nevi removed during the winter was 1.65 +/- 0.32%, whereas the proliferation fraction of the nevi removed during the summer was 1.95 +/- 0.42% (p = 0.41). For nevi from sun-exposed sites only, the proliferation rate of nevi excised during the winter was 1.81 +/- 0.39%, and 2.58 +/- 0.39% for nevi excised during the summer (p = 0.11). For nevi from sun-protected sites, there was no difference between winter and summer. When nevi excised during the summer were compared by site, sun-exposed nevi had a proliferation rate of 2.60 +/- 0.48% (p = 0.04). For nevi excised during the winter, there was a much smaller difference between sun-exposed and sun-protected nevi.