The main purpose of the study was to estimate the prevalence of psychiatric diagnoses in children with mental retardation (MR) (IQ≤70). All children born between 1980 and 1985 (N=30037) in Akershus County, Norway, were screened for possible MR and assessed with either IQ tests or standardized developmental tests. A total of 178 children, 79 with severe mental retardation (SMR) (IQ<50) and 99 with mild mental retardation (MMR) (IQ 50 to 70) were included for further study. Psychiatric symptomatology was assessed as a standard part of the neurodevelopmental examination, which included a semistructured parent interview, a clinical child interview, and retrieval of the charts of previous child psychiatric examinations. Psychiatric diagnoses were classified according to the International Classification of Disease (ICD-10). In total, 65 (37%) of the total population with MR (95% confidence intervals 29 to 44) were registered to have psychiatric diagnoses, the most common being hyperkinesia (n=28) and pervasive developmental disorder (n=15). Psychiatric diagnoses were present in 42% of the population with SMR and 33% of the population with MMR (p=0.4). Of all children found to have a psychiatric diagnosis, approximately one-third had previously been examined by a child psychiatrist and indicated a previously unrecognized need for these services to children with MR.