A meta-analysis of studies of mammographic patterns (using the definitions proposed by John Wolfe in 1976) and breast cancer risk was undertaken in order to estimate summary relative risks for women of all ages combined and separately for younger and older women. In order of increasing risks the patterns are as follows: N1 (no ducts visible), P1 (prominent ducts less than one quarter of breast), P2 (prominent ducts more than one quarter of breast), and DY (dysplasia). Women with either a P2 or a DY pattern have approximately a twofold increase in breast cancer risk compared to women with an N1 pattern, with the effect being somewhat higher among younger women. The relative risk estimates for women of all ages combined and for younger women, though not for older women, tended to increase with the number of methodologic standards met. © 1993.