The histogenesis of phyllodes tumour (PT) and that of fibroadenoma (FA) of the breast appear to be closely related. FA is thought to be hormonally responsive, while the hormone-responsiveness of PT is uncertain. To gain insight into hormone-responsiveness of PT, we performed immunohistochemical analysis of oestrogen-regulated pS2 and androgen-regulated prostate-specific antigen (PSA) protein expression and also of oestrogen receptor (ER), progesterone receptor (PgR) and androgen receptor (AR) expression in paraffin sections obtained from 50 female PT patients. Paraffin sections taken from 50 female fibroadenoma (FA) patients were analysed for comparison. ER, PgR, pS2, AR and PSA expression were detected in 32%, 96%, 20%, 98% and 4.0% of PT sections and in 28%, 96%, 42%, 80% and 10% of FA sections, respectively. No correlations were detected among ER, PgR and pS2 expression or between AR and PSA expression in PT or FA sections. PgR expression was significantly associated with AR expression in PT (P<0.0001). The present investigations indicate that PT and FA have almost similar hormone receptor status. However, different positivities of pS2 expression suggest that oestrogen-responsiveness may differ between PT and FA. In addition, a wide-ranging co-expression of AR and PgR in PT sections suggests that these receptors may play an important part in the proliferation, although the functional significance of these receptors should be elucidated.