A history of child sexual victimization may be much more prevalent among substance abusers than previously suspected and may be commonly missed if not assessed directly in every patient. The rates of reporting child sexual abuse among inpatient substance abusers were compared before and after the question was routinely asked in a treatment program. Before routine inquiry, 4% of men and 20% of women disclosed such abuse but after routine inquiries began the rates for adult men quadrupled, up to 42% of the teenaged boys reported such abuse, about 75% of adult women admitted such abuse, and 71 to 90% of teenaged girls disclosed histories of child sexual abuse. Unresolved issues from childhood sexual abuse may be a hidden factor underlying much substance abuse and if not treated may lead to rapid relapse.