Background. There is a need for more knowledge on the effects of light room treatment in patients with Seasonal Affective Disorder (SAD) and to explore patients´ subjective experience of the disease and the treatment.Methods. This was a descriptive and explorative study applying qualitative content analysis. A purposeful sample of 18 psychiatric out- patients with a major depressive disorder with a seasonal pattern according to the DSM-IV and a pre-treatment score >12 on the 9-item Montgomery-Åsberg Depression self-rating scale was included (10 women and 8 men, aged 24-65 years). All patients had completed light room treatment (> 7/10 consecutive weekdays). Data was collected two weeks post-treatment using a semi-structured interview guide.Results. Patients described a profound struggle to adapt to seasonal changes during the winter. Everyday life was affected with reduced work capacity, social withdrawal and disturbed relations with family and friends. Different individual signs and symptoms marked the onset of the seasonal depression. The light room treatment resulted in a radical and rapid change for the better, with only mild and transient side effects. Patients worried about the risk of not being able to receive the light room treatment in future possible relapses.Discussion. The patients experienced a clear seasonal pattern with deterioration in sleep, daily rhythms, energy level, mood, activity and cognitive functioning. Light room treatment was described as one of the most effective and personally important coping strategies available, with improvement in all the major symptoms. The results indicate that light room treatment is essential for some patients´ ability to cope with winter depression.