Schizophrenic patients with an early age at onset of illness had low baseline levels of homovanillic acid (HVA) in serum compared with schizophrenic patients with a late age at onset. After adjustments were made for age at onset, there was a significant partial correlation between positive symptoms and serum HVA. The relationship between positive symptom scores and serum HVA was shifted to the left in the early onset patients, suggesting a relatively increased sensitivity of dopamine-associated response. Patients with severe negative symptoms also had an earlier age at onset and a trend toward lower serum HVA. This study found no difference between mean serum HVA values in schizophrenic patients and normal control subjects.