Factors influencing fetal and childhood growth may affect a person's risk of developing schizophrenia. Associations of size at birth and body size in young adulthood with schizophrenia and other nonaffective psychoses were assessed in a cohort of 334,577 Swedish male conscripts born in 1973-1980 for whom linked birth, census, hospital admission, and adult height and weight data were available. Complete data on all study variables were available for 246,655 subjects. Over a mean 3.4-year follow-up beginning at age 18 years, 80 subjects developed schizophrenia and 124 developed other nonaffective psychoses. A reverse J-shaped association was found between gestation-adjusted birth weight and schizophrenia. The hazard ratios were 7.03 (95% confidence interval: 1.59, 31.10) for males of low birth weight (<2.5 kg) and 3.37 (95% confidence interval: 1.68, 6.74) for those of high birth weight (>4.0 kg). Birth weight was not strongly related to other nonaffective psychoses. Taller males had a reduced risk of psychosis. The lowest risks were seen for low birth weight males who became tall adults. The associations with birth weight indicate that fetal exposures, including possible effects of gestational diabetes, are important in the etiology of psychosis. The role of childhood exposures, as indexed by adult height and body mass index, appears to be less strong.