This paper gives the incidence in the Birmingham Regional Cancer Registry (England) of a second primary tumour in the contralateral breast among nearly 22,000 patients registered with a first primary in the breast between the years 1936 and 1964. The results, based on more than 90,000 women-years at risk and 399 second primary tumours, are presented with reference to 2 methods of analysis. In assessing risk, the principal factors investigated were age at first and second primary diagnoses and the interval between diagnoses. The results are discussed in terms of current aetiological hypotheses. On the basis of a method which included coincidental tumours, the overall risk of a tumour in the contraleteral breast was found to be 3.0 times that in the general population of a first primary. The corresponding risks for 3 main age-ranges (at the time of diagnosis of the first primary tumour) were 5.6 (ages 15-44 years), 3.7 (45--59 years) and 1.8 (60+ years). When coincidental tumours were excluded from the analysis, the relative risk was found to be 2.4 overall and 5.3, 3.0 and 1.0 for the 3 age-ranges, respectively. The level of risk was negatively correlated with age at first primary and the relative risk remained substantially constant over time.