This paper examines the extent to which the traditional practice of sexual abstinence during lactation has broken down among Yoruba women residents in urban areas. The first major finding is that there is a gradual erosion of the tradition, and the dominant factors of modernization are education of the woman and the use of contraception. The second major finding is that the breakdown of postpartum sexual taboos has statistically significant negative consequences on duration of lactation, although the negative impact of woman's education is greater. The third major finding is that duration of breastfeeding reduces birth interval significantly only when it is less than 15 months, and that both durations of breastfeeding and birth intervals have declined over time. The first two findings suggest further reductions in the proportion of women who abstain from sexual relations during lactation and in durations of breastfeeding as more women become more educated. Significant declines in birth intervals may follow soon after.