Residual symptoms are common in depression, and their presence is associated with poorer clinical outcomes of depression. We conducted a case series study of first-onset major depression to elucidate the clinical course of residual insomnia and examine the relationship between residual insomnia and recurrence of depression. Subjects were 128 patients (57 males; mean age 52.8 years) with first-onset major depression. For all patients, we continuously assessed the number and breakdown of residual symptoms listed on the 17-item Hamilton Rating Scale for Depression and quantities of prescribed psychotropic medications during the depressive and remission phases. Even during the first remission phase, 85.9% of the patients with first-onset major depression experienced an average of 2.95 residual symptoms. The most common residual symptom was insomnia (65.4%), followed by reduced work and interests (43.3%) and fatigue (39.4%). Each additional recurrence resulted in a significantly shorter remission phase as well as significant increases in antidepressant and hypnotics dosages. Hypnotics dosage during the first remission phase for patients with three or more recurrent episodes was significantly higher than that for those with only a single episode. Our findings suggest a possible link between treatment-resistant residual insomnia during the first remission phase and recurrence risk of depression. In particular, it is possible that presence of treatment-resistant insomnia during the first remission phase is related to later recurrence of depressive episodes. It is important to see patients with treatment-resistant insomnia of early stage carefully, with special attention to treatment adherence.