Hard keratins are expressed in normal hair and nails, and are characterized by a higher cysteine content than cytokeratins. Previous studies have suggested a coexpression of hard keratins and cytokeratins in pilomatrixoma, a benign follicular tumour which could originate from the hair matrix. Human hair keratin basic 1 (hHb1) is a newly characterized hair keratin which is expressed specifically by cortical cells of the normal hair shaft. A preliminary study has suggested that hHb1 could be expressed in pilomatrixoma. In order to confirm this hypothesis, we have studied a series of 128 pilomatrixomas by in situ hybridization, using a 35S-labelled hHb1-specific probe. The anti-sense probe was used as a negative control. Among these pilomatrixomas, six were early cases, 60 were classified into the intermediate stage (either fully developed or early regressive cases) and 62 were late regressive tumours made of shadow cells only. Forty-seven tumours showed hHb1 expression (37%), all being intermediate stage pilomatrixomas. The areas positively stained by the probe were band-like structures made of transitional cells only, which were very close to cells showing tricholemmal keratinization features. Neither the basophilic matrix cells nor the shadow cells expressed hHb1. Our results suggest that pilomatrixomas can differentiate towards cortical cells during their maturation process, as this keratin is specifically expressed in the cortex of the normal hair shaft. These data are consistent with previous studies which showed the expression of a hard keratin group in transitional cells by immunohistochemistry. The histogenesis of basophilic cells of pilomatrixoma is controversial, but it is likely that transitional cells represent an equivalent of the hair cortex.