A prospective study based on the U.S. National Collaborative Perinatal Project and using the Wechsler Intelligence Scale for Children (WISC) found lower test scores for the Coding subtest in preschizophrenic children than in their unaffected siblings. Using data on cognitive functioning in adolescence, the aim of the present prospective study was to examine whether low scores on Coding is associated with the risk of developing schizophrenia spectrum disorders. The 12 subtests of the WISC were administered to 311 children and adolescents with a mean age of 15.1 years (range: 8 to 20 years), and the diagnostic assessment (DSM-IIIR) was conducted by senior clinicians 25 years later. The group with schizophrenia spectrum disorder consisted of 84 individuals, and this group obtained significantly lower scores on Coding than nonschizophrenic controls. This difference could not be explained by differences in WISC IQ. Logistic regression analysis controlling for age at examination, gender, and social status yielded a significant, but relatively weak, association between low Coding test score and risk of schizophrenia spectrum disorder. For each unit increase in the Coding raw score, the adjusted odds ratio was 0.97 (95% CI 0.94-1.00) (p = .022), and the risk of schizophrenia spectrum disorder decreased by 3% (95% CI 6 to 0%). The Coding deficit on the WISC may indicate deficits in perceptual motor speed or in working memory processing speed in young individuals who later develop schizophrenia, schizotypal personality disorder, or other disorders within the schizophrenia spectrum.