Objective: Inflammatory processes may play a role in the pathophysiology of schizophrenia. The aim of this study was to determine the efficacy of adjuvant treatment with aspirin (acetylsalicylic acid) in schizophrenia spectrum disorders. Method: This randomized, double-blind, placebo-controlled study was conducted between May 2004 and August 2007. Seventy antipsychotic-treated inpatients and outpatients from 10 psychiatric hospitals in The Netherlands with a DSM-IV-diagnosed schizophrenia spectrum disorder were included. Patients were randomized to adjuvant treatment with aspirin 1000 mg/d or placebo. During a 3-month follow-up, psychopathology was assessed with the Positive and Negative Syndrome Scale (PANSS). Other assessments included cognitive tests and immune function. The primary efficacy outcome was the change in total PANSS score. Secondary outcomes were changes in the PANSS subscales and cognitive test results. Results: Mixed-effect models showed a 4.86-point (95% CI, 0.91 to 8.80) and 1.57-point (95% CI, 0.06 to 3.07) larger decrease in the aspirin group compared to the placebo group on the total and positive PANSS score, respectively. Similar but not statistically significant results were observed for the other PANSS subscale scores. Treatment efficacy on total PANSS score was substantially larger in patients with the more altered immune function (P = .018). Aspirin did not significantly affect cognitive function. No substantial side effects were recorded. Conclusion: Aspirin given as adjuvant therapy to regular antipsychotic treatment reduces the symptoms of schizophrenia spectrum disorders. The reduction is more pronounced in those with the more altered immune function. Inflammation may constitute a potential new target for antipsychotic drug development. Trial Registration: controlled-trials.com Identifier: ISRCTN27745631.