Within the Nijmegen breast cancer screening programme a study was made of the relation between mammographic breast features according to Wolfe's classification and breast cancer risk. The time of follow-up was 6 years. The Nijmegen study challenged the strong relationship previously suggested. In comparison with women of N1 and P1 breast type, the relative risk for women with P2 and DY breast type was estimated at 0.7, with 95% confidence limits of 0.2 and 2.4, after adjustment for the major risk factors. The results could not be explained by intra-observer variability or left-right difference.