AbstractThis study investigates themdm2 gene status and expression in 66 surgically resected human breast carcinomas, with correlations with clinico‐pathological and biological data (histological type, grading, steroid receptor status, p53 expression, proliferative activity). Four (7.7 per cent) out of 52 informative cases bearmdm2 gene amplification (four‐ to ten‐fold) and 8 (15.4 per cent) of 52 cases showed borderline amplification (three‐fold). Nine (13.6 per cent) out of 66 cases showed strong mdm2 nuclear immunoreactivity. Twenty‐seven (40.9 per cent) cases showed isolated mdm2 reactive nuclei. All cases with clear amplification showed a high percentage of mdm2 immunoreactive nuclei. The relationship between gene amplification and mdm2 protein expression is highly significant (P<0.0001). No association was observed betweenmdm2 gene amplification and any of the considered clinico‐pathological and biological parameters, while mdm2 immunoreactivity showed a significant association only with oestrogen receptor immunoreactivity (P=0.009). p53 expression was associated neither withmdm2 gene amplification nor with mdm2 immunoreactivity. It could be tempting to hypothesize that the evaluation of the combined mdm2/p53 immunohistochemical phenotype in human breast carcinoma could give us better prognostic information than the evaluation of the expression of the p53 pr