Evidence is accumulating that developmental abnormalities, poor neuromotor function, and increased problem behavior precede the manifestation of schizophrenia itself. Information on school performance and behavioral development was obtained for 49 pairs of twins in which at least one twin suffered from schizophrenia (20 monozygotic [MZ], 29 dizygotic [DZ]) and for 43 pairs of healthy control twins (25 MZ, 18 DZ). Cox regression was used to analyze the contribution of schizophrenia, zygosity, sex, and year of birth to the age at which developmental divergence occurred. In both MZ and DZ twin pairs with schizophrenia, divergence in school performance occurred about 7.5 years earlier than it did in control twins, at 12 years of age, preceding the onset of psychosis by 10 years. This suggests that the first prodromal signs of schizophrenia manifest themselves as cognitive symptoms at the onset of puberty. Underperformance at school might therefore be considered one of the first signs of an until then latent vulnerability for schizophrenia.