There has been considerable interest in the relation between familysize desires and completed fertility. Longitudinal data from the Princeton Fertility Study provide a unique opportunity to compare the number of children desired after the birth of a second child to the size of completed families. The average number of children desired by women after the birth of their second child predicts very well the average size of their completed families. The average family size desired at the first interview and average estimated completed family size some eight years later are identical for the total sample, and vary hardly at all within religious or education subgroups. One-third of the variance in the completed fertility of couples is “explained” by wife’s family-size desires and the proportion of explained variance increases to two-fifths when we include the husband’s first interview desires and the interval between marriage and second birth. Yet in spite of the relatively high correlation between desires and achievement, only 41 percent of these women achieved exactly the number of children they desired at the first interview, while 14 percent had two children more or fewer than originally desired. Nevertheless, data on contraceptive efficiency indicate that desires after the birth of the second child constitute meaningful goals in terms of which the respondents regulate their subsequent behavior—efficacy of contraceptive practice shows substantial improvement after the desired number has been achieved.