The aims of this study were to use longitudinal report data on physical and sexual abuse to examine the stability and consistency of abuse reports. The study was based on the birth cohort of young people studied in the Christchurch Health and Development Study. At ages 18 and 21 years, these young people were questioned about their childhood exposure to physical punishment and sexual abuse. Concurrent with these assessments, sample members were also assessed on measures of psychiatric disorder and suicidal behaviour. Reports of childhood sexual abuse and physical punishment were relatively unstable and the values of kappa for test-retests of abuse reporting were in the region of 0.45. Inconsistencies in reporting were unrelated to the subject's psychiatric state. Latent class analyses suggested that: (a) those not abused did not falsely report being abused; and (b) those who were abused provided unreliable reports in which the probability of a false negative response was in the region of 50%. Different approaches to classifying subjects as abused led to wide variations in the estimated prevalence of abuse but estimates of the relative risk of psychiatric adjustment problems conditional on abuse exposure remained relatively stable. There was substantial unreliability in the reporting of child abuse. This unreliability arose because those who were subject to abuse often provided false negative reports. The consequences of errors in reports appear to be: (a) that estimates of abuse prevalence based on a single report are likely to seriously underestimate the true prevalence of abuse; while (b) estimates of the relative risk of psychiatric adjustment problems conditional on abuse appear to be robust to the effects of reporting errors.