It is hypothesised that the risk of schizophrenia may be elevated in children conceived following a short inter-pregnancy interval, when maternal folate stores are still being replenished. We examined the relationship between inter-pregnancy interval and schizophrenia risk in a longitudinal, population-based cohort. Risk of schizophrenia was increased by approximately 150% in those born following a pregnancy interval of ≤6 months, but was not increased if the interval after birth of the participant, before conception of the subsequent sibling, was ≤6 months. These findings support the hypothesis that folate (or other micronutrient) deficiency during fetal development may be an important risk factor for schizophrenia.