Delayed childhood development may precede adult psychoses. We tested this hypothesis in a large, general population birth cohort (n=12058) followed to age 31 years. The ages at which individuals learned to stand, walk, speak, and became potty-trained (bowel control) and dry (bladder control), were recorded at a 1-year examination. Psychiatric outcome was ascertained through linkage to a national hospital discharge register. Cumulative incidence of DSM-III-R schizophrenia, other psychoses and non-psychotic disorders were stratified according to the timing of milestones and compared within the cohort using internal standardization. 100 cases of DSM-III-R schizophrenia, 55 other psychoses, and 315 non-psychotic disorders were identified. The ages at learning to stand, walk and become potty-trained were each related to subsequent incidence of schizophrenia and other psychoses. Compared with the whole cohort, earlier milestones reduced, and later milestones increased, the risk in a linear manner. These developmental effects were not seen for non-psychotic outcomes. The findings support hypotheses regarding psychosis as having a developmental dimension with precursors apparent in early life.