CONTEXT: Iron is essential for brain development and functioning. Emerging evidence suggests that iron deficiency in early life leads to long-lasting neural and behavioral deficits in infants and children. Adopting a life course perspective, we examined the effects of early iron deficiency on the risk of schizophrenia in adulthood. OBJECTIVE: To determine whether maternal iron deficiency, assessed by maternal hemoglobin concentration during pregnancy, increases the susceptibility to schizophrenia spectrum disorders (SSDs) among offspring. DESIGN: Data were drawn from a population-based cohort born from 1959 through 1967 and followed up for development of SSD from 1981 through 1997. PARTICIPANTS: Of 6872 offspring for whom maternal hemoglobin concentration was available, 57 had SSDs (0.8%) and 6815 did not (99.2%). MAIN OUTCOME MEASURE: Prospectively assayed, the mean value of maternal hemoglobin concentration was the primary exposure. Hemoglobin concentration was analyzed as a continuous and a categorical variable. RESULTS: A mean maternal hemoglobin concentration of 10.0 g/dL or less was associated with a nearly 4-fold statistically significant increased rate of SSDs (adjusted rate ratio, 3.73; 95% confidence interval, 1.41-9.81; P = .008) compared with a mean maternal hemoglobin concentration of 12.0 g/dL or higher, adjusting for maternal education and ethnicity. For every 1-g/dL increase in mean maternal hemoglobin concentration, a 27% decrease in the rate of SSDs was observed (95% confidence interval, 0.55-0.96; P = .02). CONCLUSIONS: The findings suggest that maternal iron deficiency may be a risk factor for SSDs among offspring. Given that this hypothesis offers the potential for reducing the risk for SSDs, further investigation in independent samples is warranted.