This prospective, longitudinal study investigated the frequency and pattern of mood changes between outpatients receiving usual care for bipolar disorder who were either taking or not taking antidepressants. Eighty patients with bipolar disorder self-reported mood and psychiatric medications daily for 3 months using a computerized system (ChronoRecord) and returned 8662 days of data. Of the total group of 80 patients, 47 took antidepressants; 33 did not. Patients taking antidepressants reported depression twice as frequently (29% of days vs. 13.8% of days). In both groups, two-thirds of all mood changes over a 1-, 2- and 3-day period were small, between -5 and 5 on a 100-point scale. No statistically significant difference was found in the frequency of large mood changes (>10 on a 100-point scale) or in switches between depression and mania (0.7% if not taking antidepressants vs. 0.9% if taking), independent of diagnosis of bipolar I or II. Eighty-nine percent of patients taking antidepressants were also taking mood stabilizers. In this naturalistic setting, no significant difference between the rate of switches to mania or rapid cycling was found between those taking and not taking antidepressants, regardless of diagnosis. The primary difference in pattern between the groups was the time spent in depressed or normal mood, with minor daily mood variations.