Since 1969, 4-year-olds in the County of Uppsala have been offered extensive health screening at the Child Health Centres. In 1969 and 1970 a total of 3810 children underwent the screening procedure. The psychological screening instrument consisted of a questionnaire (to be answered by the parents), an interview of the parents, a psychomotor examination and an observation of the child's behaviour. 156 (4.1%) 4-year-olds were referred to a specialist team for investigation and decision concerning treatment. In the present study the effectiveness of the psychological screening instrument was assessed by its sensitivity, specificity and predictive value. The sensitivity was found to amount to a maximum of 0.73. The specificity was estimated to be 0.98-0.99, and the positive predictive value to 0.67. The authors consider the effectiveness to be sufficiently high to motivate continuation of the screening procedure for detection of psychological problems in pre-school age. Efforts should be made, however, to further improve the effectiveness.