We investigated whether observable behavior of seasonal affective disorder (SAD) patients and an interviewer during an interview before light treatment is related to the response to the light treatment. Different observed behavioral elements of 24 SAD patients and of 2 interviewers, assessed before light treatment, were reduced to "behavioral factors." Forward multiple regression analyses were applied to investigate whether these factors might predict the response to light therapy (3 h of bright light between 09:00 and 12:00 h or between 18:00 and 21:00 h on 5 consecutive days). In addition, it was investigated whether the interviewers' factors could be predicted from the patients' factors. Both patients' and interviewers' factors predicted the response to light treatment. Response-related factors of patients and interviewers were interrelated. The results suggest that behavioral processes may play a role in the mechanisms underlying the response to light treatment in SAD. They support the relevance of interpersonal theories in seasonal depression.