Sex differences in practice patterns, as modified by family roles, are investigated in a national survey of 1420 active obstetrician-gynecologists who graduated from medical school between 1974 and 1979. Women are more likely than men to be practicing in multispecialty groups, and men are more likely than women to be practicing in obstetrics-gynecology partnerships. On average, men and women report working over 60 hours per week. In all practice arrangements except academic medicine, women work fewer total hours per week, although the differences are small and translate into significantly fewer patient encounters than men in only two practice arrangements: partnerships and multispecialty groups. When marital status and presence of children under age 18 are controlled, significant sex differences in hours worked remain only for married respondents with children. Family roles have an opposite effect on hours of work reported by men and women, decreasing the number of hours worked by women and increasing the number worked by men.