This study evaluates the effects of potential risk factors for benign breast disease (BBD) with special attention to the histologic and mammographic specificity of the effects. Cases were 172 women with BBD that underwent biopsy; controls were 134 women free of breast signs or symptoms. All cases and controls had undergone mammography. For all types of BBD combined, parity, use of oral contraceptives, and use of exogenous estrogen after menopause were strongly protective, whereas obesity and early menarche were weakly protective. Family history of breast cancer was virtually unrelated to BBD. The protective effect of parity was stronger for BBD with intralobular or extralobular fibrosis, and with mammographic homogeneous density or large nodular densities, than it was for BBD without these characteristics. Similar relations with the histologic and mammographic features were observed for obesity. These findings suggest that some risk factors for BBD have effects that are related to specific features of its morphology.