The aim of this study is to test the hypothesis that there is a depletion of polyunsaturated fatty acids of erythrocyte membranes in patients with bipolar disorder and to connect the previous therapeutic and psychoimmunological findings. Fatty acid compositions of erythrocyte membranes in 20 bipolar manic patients and 20 healthy controls were analyzed by thin-layer chromatography and gas chromatography. The major finding was significantly reduced arachidonic acid (20:4n-6) and docosahexaenoic acid (22:6n-3) compositions in bipolar patients as compared to normal controls with P values of 0.000 and 0.002, respectively. There were no differences in total omega-3 and omega-6 polyunsaturated fatty acids. This abnormality may be related to the mechanisms of action of mood stabilizers and the previous findings on the abnormal psychoimmunology of patients with bipolar disorder. Larger sample sizes of medicated patients or drug-free manic, well-controlled designs on the diet and smoking, and fatty acid composition measurements during full remission after the index episode are warranted in future studies.