A response to treatment and long-term course of 14 adults with delayed sleep phase syndrome were investigated with the use of their hospital records and mailed questionnaires. Six patients treated with chronotherapy showed full recovery just after the treatment. In three of them the delay of sleep phase relapsed one year afterwards. Four of five patients treated with pharmacotherapy alone showed partial recovery. Six of nine patients followed for periods of longer than three years after treatment showed good prognoses, though all of them still had a mild phase delay and had to shorten their sleep time in order to work full-time. The three others had poor prognoses and one of them was under psychiatric treatment for affective instability. These findings suggest that a long-term follow-up is needed to judge the effect of the treatment.