The association between the frequency of meat and fish intake and the incidence rate of breast cancer has been analyzed in 152 incident cases that developed among 14,500 Norwegian women during 11 to 14 years of follow-up. At the time of dietary inquiry they were between 35 and 51 years of age. A positive association was observed between the frequency of overall meat intake and breast cancer risk. There was an age-adjusted incidence rate ratio (IRR) of 1.8 (95% confidence limits, 1.1 and 3.1) for women who had a main meal with meat 5 times or more per week compared to women who had 2 meat dinners or less per week, and this association displayed a linear trend (chi 2 trend = 4.30, p = 0.04). No association was detected between the overall frequency of fish for dinner and breast cancer risk (chi 2 trend = 1.39, p = 0.24), but there was an inverse relation with the frequency of main meals containing fish in poached form. The age-adjusted IRR was 0.7 (95% confidence limits, 0.4 and 1.0) for women who had poached fish for dinner at least 5 times per month compared to women who had fish in this form twice monthly or less often (chi 2 trend = 3.56, p = 0.06). The positive association with meat may be in accordance with the hypothesis that dietary fat increases the risk of breast cancer. Although there was no association with overall fish intake, the inverse relation with poached fish might deserve further investigation.