Hypnic headache syndrome is a rare, sleep-related, benign headache disorder. We report 19 new cases (84% females) with follow-up data. The mean age at headache onset was 60.5 +/- 9 years (range 40-73 years). Headache awakened the patients from the night's sleep at a consistent time, usually between 1.00 and 3.00 a.m. (63%); three patients (16%) reported that identical headaches could occur also during daytime naps. Headache frequency was high, occurring more than 4 nights/week in 68% of the patients. Headache resolution occurred within 2 h in 68% of patients. Neurologic examination, laboratory studies, and brain imaging were unrevealing at the time of diagnosis. Headache severity largely remains unchanged or attenuates over time, but frequency may vary in either direction. Only one patient had spontaneous relief from headache. Four patients (24%) achieved permanent suppression of headache with medication, and two were able to abort individual headache attacks. Caffeine in a tablet or beverage was helpful in four patients. Lithium carbonate therapy caused side effects requiring cessation of treatment in four patients.