The effect of marijuana on bipolar disorder has never been systematically evaluated. Subjective reports by patients suggest an overall positive effect, but these may be unreliable. We herein report a case in which mood data was prospectively collected over two years of total substance abstinence and two years of extreme marijuana use. Marijuana use did not alter the total number of days of abnormal mood, however, marijuana was associated with an increase in the number of hypomanic days and a decrease in the number of depressed days. While not conclusive, the data suggest that marijuana may indeed have an effect on mood in bipolar patients that needs to be systematically examined.