The aim of this study is to examine the efficacy of omega-3 fatty acids in the form of eicosapentaenoic ethyl esters (EPA) - in the treatment of bipolar disorder. Omega-3 fatty acids are long-chain, polyunsaturated fatty acids found in plant and marine sources. There are preliminary data to suggest that omega-3 fatty acids may be efficacious in the treatment of mood disorders. Patients in our bipolar outpatient clinic will be at one of the five NIMH - Stanley Foundation Bipolar Network (the "Network") sites participating in this project. Subjects will be randomly assigned in a double-blind manner to 6 grams per day of omega-3 fatty acids or placebo (paraffin capsules) as an "add-on" to ongoing treatment with mood stabilizing medication(s) which have proven unsatisfactorily effective within therapeutic range(s) or at maximum tolerated dose(s). The double-blind trial will continue for 4 months duration. Patients will then be offered the option of entering an eight month, open-label trial of omega-3 fatty acid. The hypothesis of this study is that omega-3 polyunsaturated fatty acids in the form of eicosapentaenoic acid (EPA) acting on some of the same signal transduction mechanisms as the mood stabilizers will be beneficial in breakthrough depression, mania and cycling of bipolar disorder.