BACKGROUND A substantial proportion of patients with bipolar disorder remain symptomatic during inter-episode periods, and mood instability is associated with high risk of relapse and hospitalization. Few studies have investigated long-term daily illness activity and none has compared bipolar type I and II using daily data. The objectives were to investigate differences in daily illness activity between bipolar disorder type I and II. METHODS A smartphone-based system for self-monitoring was developed. A total of 33 patients treated in a mood clinic used the system for daily self-monitoring during a median period of 310 days [IQR 189; 437]. Data presented summarize over 8500 observations. RESULTS Patients with bipolar disorder type II (n=20), compared to patients with bipolar disorder type I (n=13), experienced a significant lower mean level of mood on a scale from -3; +3 (-0.54 (95% CI: -0.74; -0.35) versus -0.19 (95% CI: -0.35; -0.02), p=0.02), less time euthymic (51.0% (95% CI: 36.4; 65.7) versus 74.5% (95% CI: 62.4; 86.7), p=0.03) and a higher proportion of time with depressive symptoms (45.1% (95% CI: 30.6; 59.5) versus 18.8% (95% CI: 6.9; 30.7), p=0.01). The proportion of time spent with (hypo)manic symptoms did not differ (2.7% (95% CI: 0.1; 5.5) versus 5.5% (95% CI: 3.1; 7.8), p=0.17). LIMITATIONS Patients received different types, doses and combinations of psychopharmacological treatment. CONCLUSION Euthymia was obtained for a substantial proportion of time in patients with bipolar disorder type I, but despite on-going treatment only for half of the time for patients with bipolar disorder type II. This emphasizes the need for improving treatment strategies for bipolar disorder type II.