It was hypothesized that pre-treatment activation plays a role in the response to light therapy in Seasonal Affective Disorder (SAD). In 55 SAD patients (DSMIII-R) energetic and tense activation was assessed before light therapy via self-rating (AD-ACL) and voice sound characteristics (mean pitch and variation in pitch). These variables were studied in relation to the 'acute' response to 4 days of light therapy (30 min, 10000 lux) and to a 'late' response (11 (10) days after light therapy had stopped). Acute response was defined as the percent change in 3 times daily self-rated depressed mood (AMS) with respect to the average of 4 baseline days. 'Late' response was defined as the percent change in HRSD or AMS scores between baseline and 11 (10) days after light therapy. It was found that patients having high pitched voices with small variation in this pitch benefitted more from light therapy than the patients with low pitch and large variation in pitch levels. This effect was only significant after the first day of light exposure. No other significant relations were found between baseline activation and acute or late responses to light therapy. Hence, light therapy seems to give extra comfort in 'tense' patients, who become rapid responders to light therapy.