Data obtained from a random sample of 930 adult women in San Francisco provide the soundest basis heretofore available for estimating the prevalence of intrafamilial and extrafamilial sexual abuse of female children. This article describes the methodology of this survey, as well as some of the key findings. For example: 16% of these women reported at least one experience of intrafamilial sexual abuse before the age of 18 years; 12% reported at least one such experience before the age of 14 years; 31% reported at least one experience of extrafamilial sexual abuse before the age of 18 years; and 20% reported at least one such experience before the age of 14 years. When both categories of sexual abuse are combined, 38% reported at least one experience before the age of 18 years; and 28% reported at least one such experience before the age of 14 years. Only 2% of the cases of intrafamilial and 6% of the cases of extrafamilial child sexual abuse were ever reported to the police. A plea is made for the urgent need to recognize the magnitude of the problem of child sexual abuse, and to act to prevent it.