The present prospective high-risk study examined associations between childhood scores on five Wechsler Intelligence Scale for Children (WISC) subtests (vocabulary, similarities, block design, object assembly, and mazes) and later development of schizophrenia spectrum disorders (SSD). The sample comprised 244 high-risk or control children who were administered the WISC subtests at age 10 to 13 years in 1972. Adult psychiatric data were gathered from psychiatric interviews in 1992-93 and from the Danish Psychiatric Central Register in 2007. Thirty-two participants had developed SSD, 79 other psychiatric disorders (OPD), and 133 had no diagnosis (ND). The SSD group obtained lower scores than the ND group on all subtests and IQs, but when adjusted for sex and parental social status only significantly lower scores on similarities, object assembly, mazes, and total IQ. Compared with the ND group, the OPD group obtained significantly lower scores on similarities, vocabulary, verbal IQ, and total IQ. The only significant difference between the SSD and OPD groups was on object assembly (OPD performed at the level of ND). The results suggest a premorbid deficit in general intelligence in individuals who later develop SSD. The results for the OPD group support recent studies demonstrating that premorbid IQ deficits may characterize a wide range of psychiatric disorders.