Immunohistochemical analysis was used to evaluate p27kip1 expression in normal hair follicles and in a series of 39 basal cell carcinomas (BCC) (13 of superficial type, 7 infiltrating, 7 morphea-like, 12 nodular) and 20 trichoepitheliomas (TE) (9 of classic type, 9 immature, 2 desmoplastic). The labeling index (LI) was derived semiautomatically by means of a computer-assisted cellular image analyzer, and statistical analysis was carried out using the Student t test. A positive reaction for p27kip1 was detected in the hair germ papillae, in supramatrical cells, and in the inner pilar sheath, whereas matrical cells and the outer pilar sheath were negative. All BCC and TE cases showed a positive immunoreaction for p27kip1, but the staining pattern was different in the two groups of lesions, being patchy with focal peripheral accentuation in TE and more diffusely dispersed in BCC. The quantitative study showed lower p27kip1 expression in BCC (LI = 27.51 +/- 12.55) than in TE (LI = 45.27 +/- 20.27) (P < 0.0001). Statistically significant differences were also observed between TE subgroups and nodular or infiltrating BCC subtypes. The occurrence of a wide overlap of LI values hampers the practical application of a p27kip1 LI in the differential diagnosis between BCC and TE in difficult cases, however.