The relationship of marital and reproductive experience to human breast cancer was studied using data collected during 1956-1962 by the Department of Biostatistics and Epidemiology at the Roswell Park Memorial Institute in Buffalo, New York. Information of epidemiological interest was obtained from all women with cancers of the reporductive organs in Buffalo, New York, and the adjoining township of Kenmore. For comparison, a probability sample of the same population was selected. During 1965-1967, an International Collaborative Study with similar objectives found a striking positive relationship between age at first birth and breast cancer risk. This finding can be interpreted as either indicating that an event associated with first birth at an early age protects against the development of breast cancer or that the hormonal status of a women both produces a delay in a woman becoming pregnant and increases the risk of developing breast cancer. The analysis of marital and reproductive histories in the Buffalo population study confirms previous reports of an increasing risk of breast cancer with increasing age at first birth. Attempts to distinguish the two interpretations mentioned by analyzing the interval between first marriage and time of birth in addition to age at first birth were not conclusive. This necessitated a similar analysis of data available in a larger series of 1164 breast cancer patients and 1200 non-neoplastic controls hospitalized at the Roswell Park Memorial Institute during 1957-1965. The results do not show an influence of interval between first marriage and first birth but do show an increased risk of breast cancer with increasing age at first birth. The results are thus consistent with a protective effect of an earyl age at first birth. These findings also have a bearing on recent suggestions that ovulatory failure predisposes to development of breast cancer.