The proliferative activity of epidermal structures was determined in 6 patients suffering from lichen planus (i.p.). By using the in vitro 3H-thymidine labelling technique, semithin Epon sections of lesional and perilesional uninvolved skin were compared autoradiographically with normal controls. The following results were obtained. The labelling index of epidermal cells was significantly increased within the i.p. papules. The mean values ranged from 41% in acanthotic i.p. to 23% in atrophic i.p. whereas in adjacent unaffected skin (15% and 11% respectively) as well as in normal controls (6%) a lower 3H-index was observed. As regards damage to basal cells within i.p. lesions, a factor--the relation between the number of basal cells in involved and uninvolved skin in reference to the epidermal length--was calculated in order to correct the autoradiographic results in involved skin. The differences between i.p. papules, adjacent skin and controls remained statistically significant even after correction. The intradermal eccrine ducts revealed a significantly increased labelling index in i.p. lesions as compared with perilesional and normal skin. In this respect no difference could be observed between the acanthotic and the atrophic variant of i.p. These findings indicate that replacement of damaged basal cells in i.p. is achieved by an increase in actively dividing keratinocytes in both epidermis and skin appendages within the lesion.