BACKGROUND: We examined cognitive deficits before and after onset of schizophrenia in a longitudinal study that: 1) covers a long time interval; 2) minimizes test unreliability by including the identical measure at both childhood and post-onset cognitive assessments; and 3) minimizes bias by utilizing a population-based sample in which participants were selected neither for signs of illness in childhood nor for being at risk for schizophrenia. METHODS: Participants in the present study, Developmental Insult and Brain Anomaly in Schizophrenia (DIBS), were ascertained from an earlier epidemiologic study conducted in Oakland, CA. The original version of the Peabody Picture Vocabulary Test (PPVT), a test of receptive vocabulary, was administered at age 5 or 9 and repeated as part of the DIBS study at an average age of 40. There were 10 DIBS cases with DSM-IV schizophrenia or schizoaffective disorder and 15 demographically similar DIBS controls with both child and adult PPVT scores. RESULTS: Cases scored significantly lower than controls in childhood (d=0.95) and adulthood (d=1.67). Residualized scores indicating the number of SDs above or below one’s predicted adult score revealed a mean case-control difference of -1.51SDs, consistent with significant relative decline over time among the cases (p<0.0013). CONCLUSIONS: In this prospective study, individuals who developed adult schizophrenia manifested impaired receptive vocabulary during childhood and further relative deterioration (or lack of expected improvement) between childhood and midlife. Limitations should also be acknowledged, including the small sample size and the fact that we cannot be certain when the continued deterioration took place.