The aim of this pilot study was to evaluate a potential skin test for schizophrenia based on the effect of aqueous methyl nicotinate (AMN) on the production of prostaglandin D2 (PGD2) from skin macrophages and the resultant cutaneous capillary vasodilatation. Four concentrations of AMN were applied topically to the forearm skin in patients and controls, and any resulting vasodilatation was rated as redness after 5 min. The test was carried out on 38 patients with schizophrenia diagnosed according to DSM-III-R criteria, and 22 normal control subjects. At all concentrations of AMN, the schizophrenics were highly significantly different from the controls. One concentration gave the greatest degree of differentiation: at this concentration at 5 min, 83% of schizophrenics but only 23% of controls had a zero or minimal response to AMN. The skin flushing seen after oral administration of nicotinic acid is due to the same reaction, and this has been normal in those with affective illness and neurosis; cyclo-oxygenase inhibitors, e.g., aspirin, give a false-positive result (failure of vasodilatation). This result is consistent with the concept of reduced membrane arachidonic acid levels in schizophrenia. This test may contribute to the reliable diagnosis of schizophrenia.