To provide more precise measurement of sexual activity and conception among U.S. teenage women than previously available, life table probabilities of 1st intercourse and of 1st conception at each age based on retrospective data collected in 2 national surveys, 1971 and 1976, of women aged 15-19 are presented. Estimates reflect the combined experience of the 5 single-year-of-age cohorts included in the 1976 study and of the 5 cohorts included in the 1971 study. Including ever-married as well as never-married, there were 2193 interviews in 1976 and 4392 interviews in 1971; both involved oversampling of blacks. Probabilities of 1st intercourse and of 1st conception are based on single-decrement life tables while estimating the probabilities of 1st premarital intercourse and of 1st premarital conception involves a double-decrement procedure. All tables are according to race, white and black, and cumulative probabilities and the proportion of conception ending in live births are also revealed. Data presented here is consistent with results published earlier from the 2 surveys showing an increase in the proportion of young women ever having premarital intercourse, a seeming improvement in the regularity of contraceptive use, and an increase in the use of more effective methods of contraception. The earlier results, however, also show no change for whites in the rate of premarital conceptions among the sexually active, and only a small decline for blacks. Among possible explanations for undiminished pregnancy despite significant shifts to more regular and effective contraception are: 1) underreporting by whites in 1971 of pregnancies terminating in abortions; 2) changes in the proportion of those sexually active who had intercourse only once; and 3) changes in the frequency of intercourse. The data to test these and other possible explanations are not available. Clearly, however, more young women aged 15-19 in 1976 had premarital intercourse than comparable women in 1971 and the cumulative likelihood of premarital intercourse among 1976 respondents was higher than the cumulative likelihood of premarital and marital intercourse in 1971.