In the large, hospital-based, international case-control study of breast cancer conducted by MacMahon and colleagues in the 1960s, no protective effect of lactation was observed. However, more recent reports have suggested that such an association may be limited to pre-menopausal women. Therefore, a re-analysis of the data from that original study was performed by menopausal status and with control for additional breast-cancer risk factors since identified. Overall, data from 4,671 parous pre-menopausal and 7,200 parous post-menopausal women were analyzed, from 7 different sites representing areas of high risk (Glamorgan, Wales; Boston, USA), moderate risk (Athens, Greece; Slovenia, ex-Yugoslavia; São Paolo, Brazil), and low risk (Tokyo, Japan; Taipei, Taiwan) of breast cancer. No significant effect of lactation was found for pre-menopausal or post-menopausal women from the high-, moderate-, or low-risk areas; the center-adjusted, combined odds ratio (OR) for having breast-fed was 1.05 (95% confidence interval, 0.86-1.29) among pre-menopausal and 1.04 (0.88-1.24) among post-menopausal women. Moreover, examination of cumulative duration of lactation did not support an inverse association between breast cancer and increased length of total breast-feeding. In conclusion, re-analysis of these data, by menopausal status and adjusting for age at first parity, age at menarche, age at menopause, body-mass index and years of schooling, did not reveal a protective effect of lactation or duration of lactation against breast-cancer occurrence among the pre-menopausal, parous women.