Adjuvant tamoxifen treatment reduces the occurrence of contralateral breast cancer (CBC). The aim of the study was to investigate the hypothesis that adjuvant tamoxifen reduces the occurrence of oestrogen-receptor (ER)-positive CBC, but not the growth of ER-negative CBCs, and to examine survival after diagnosis of CBC. For the study, ER status was immunohistochemically assessed in CBCs of 35 tamoxifen-treated patients and 115 patients without previous hormonal treatment. Cases were retrieved from a series of patients treated from 1984 to 1995 at nine hospitals. The interval between ipsi- and contralateral breast cancer was at least 1 year. It was seen that the proportion of patients with an ER-negative CBC was significantly higher among those with prior tamoxifen treatment: 37% vs 18% (P=0.047). No difference between the two groups in overall and disease-specific survival following CBC was found. However, the stage differed for both groups: tamoxifen users more often had node-positive contralateral disease (P= 0.045). In conclusion, metachronous CBCs developing after 1-3 years of tamoxifen treatment are more often ER-negative breast cancers. So far this does not seem to have a major impact on survival.