Of 2019 men and women (aged 15 years and over) interviewed as part of a MORI Survey of a nationally representative sample of Great Britain, 10% reported that they had been sexually abused before the age of 16 (12% of females; 8% of males). There was no increased risk associated with specific social class categories or area of residence. For all types of sexual abuse, the mean age of victims when first abused was significantly lower for females. Subjective reports of the effects of sexual abuse indicated that the majority (51%) felt harmed by the experience, while only 4% reported that it had improved the quality of their life. We estimate that there are over 4.5 million adults in Great Britain who were sexually abused as children, and that a potential 1,117,000 children will be sexually abused before they are 15 years of age. At least 143,000 of these will be abused within the family. The social and mental health implications are enormous, and the authors suggest that an effective intervention and prevention policy is urgently required.