The objective was to test effects of ethyl eicosapentaenoate (E-E) on persistent ongoing symptoms in patients receiving different types of anti-schizophrenic drugs, typical antipsychotics, new atypical antipsychotics, and clozapine. 115 patients with DSM-IV-defined schizophrenia were studied, 31 on clozapine, 48 on new atypical drugs and 36 on typical antipsychotics. Placebo or 1, 2 or 4 g/day of E-E was given for 12 weeks in addition to the background medication. The main assessment was change from baseline to 12 weeks on the PANSS and its sub-scales. There were no treatment-related side effects or adverse biochemical or haematological effects. Patients on 2 and 4 g/day E-E showed significant reductions in triglyceride levels which had been elevated by clozapine. In patients given 2 g/day E-E there were improvements on the PANSS and its sub-scales, but there was also a large placebo effect in patients on typical and new atypical antipsychotics and no difference between active treatment and placebo. In patients on clozapine, in contrast, there was little placebo response, but a clinically important and statistically significant effect of E-E on all rating scales. This effect was greatest at 2 g/day. There was a positive relationship between improvement on rating scales and rise in red blood cell arachidonic acid concentration.