The correlation between prolactin (PRLR) and oestrogen (ER) or progesterone receptors (PgR) in breast cancer and a possible prognostic significance of PRLR at 10 year follow-up have been investigated in the Naples (GUN) adjuvant trial. A total of 308 pre- and post-menopausal patients with early breast cancer, who entered the trial from 1 February 1978 to 31 December 1983, received randomly Tamoxifen (TM), 30 mg per die for 2 years, or no therapy. PRLR status was known in 229 (74.3%) patients. Values of specific binding less than 1% were considered negative. PRLR was positive in 75/229 (32.8%). ER was assayed in 210/229 (91.7%) patients and PgR in 188/229 (82.1%). No significant correlation, by the Spearman test, was found between PRLR and ER or PgR, while ER status was highly interrelated with PgR status. By the Cox model no evidence of an independent prognostic role of PRLR on disease-free survival (DFS) was observed, nor an interaction between PRLR and adjuvant treatment with TM was found.