A degree of ventricular enlargement, together with a reduction of total cortical mass and loss of asymmetry is reported in schizophrenia, but the meaning is obscure. These changes may reflect an anomaly of brain development. Brain structure was assessed on a 1.5-Tesla MRI scan in a series of 29 adolescents at the time of a first episode of schizophrenia and compared with 15 adolescents with other serious psychiatric disturbance (mostly psychotic) and 20 normal adolescent controls. The age at scan ranged between 13 and 20 years. In the adolescents with a diagnosis of schizophrenia, total brain volume increased with age in a way that differed significantly (p=0.007) from that seen in patients with other psychiatric disturbance and normal controls. Thus, brain growth, as assessed by this index, had reached a plateau in the control group by the age of 13 years, but this was not true of patients with schizophrenia. The measure that most clearly distinguished the groups (p<0.001 after co-varying for height and sex) was the volume of the left lateral ventricle the ventricle was significantly larger in patients with schizophrenic illness, and ventricular size increased with age to a greater extent in the patient group, although not significantly so, than in normal controls. Thus, aspects of brain growth are delayed in patients with early onset schizophrenia, and the greatest severity of illness is reflected in a component of growth that is lateralized to the dominant hemisphere. Individuals who develop serious psychiatric illness, including schizophrenia, represent a fraction of the population in whom a component of the relative development of the cerebral hemispheres occurs late.