Introduction: A recent study indicates that the serotonin transporter function varies with the duration of monthly sunshine. This finding may suggest that the efficacy of serotonin selective reuptake inhibitors (SSR1s) is associated with the duration of monthly sunshine, but information on this possible association is lacking. Methods: Ninety-one Japanese subjects with depression completed a 6-week treatment with paroxetine. Clinical evaluation was performed using the Montgomery and Asberg Depression Rating Scale (MADRS) before treatment and after 1, 2,4 and 6 weeks of treatment. Data on the duration of monthly sunshine were obtained from the meteorological agency website. We divided the patients into four groups: nonresponders (NRs), later responders (LRs), early responders (ERs) and ultra-early responders (UERs). Results: The responses to paroxetine treatment of the group that began treatment in fall and winter and of the group that began treatment in spring and summer did not differ significantly. The effect of the duration of monthly sunshine on paroxetine response time did not differ significantly among the four groups, whereas the change in the duration of monthly sunshine had a significant effect on paroxetine response time. Limitations: Our sample of patients with MDD was small and only included the Hirosaki area. Conclusion: The change in the duration of monthly sunshine is associated with paroxetine response time. (C) 2011 Elsevier B.V. All rights reserved.