Follow-up studies of idiopathic thunderclap headache (ITH) have found no subsequent subarachnoid hemorrhage (SAH) or other serious neurological disease, but the effect on life-style has not been studied. To assess the long-term outcome of patients with ITH in general practice we prospectively followed 93 patients with an episode of ITH during 1988-1993, of whom 77 were referred to hospital. ITH was defined as a sudden, unusually severe headache that started within 1 min, lasted at least 1 h, and for which no underlying cause was found. These patients were treated in 252 general practices. Outcome measures were subsequent SAH, subsequent headaches, absence from work, and diminished daily functioning. Patients were followed up by their general practitioner for a median of 5 years (range 1-10). Three patients died, all from non-neurological diseases. No subsequent SAH was diagnosed in any of the 93 patients. Recurrent attacks of ITH occurred in 8 patients, and 13 developed new tension headache or migraine. Absence from work because of headache was recorded in 11 patients, and in the overall group 6 patients were dependent on welfare. In only one-half of patients (n=52) did the general practitioner judge the level of daily functioning to be similar to that before the index episode of ITH. Thus, although no episodes of SAH occurred after ITH during long-term follow-up, one-half of patients with ITH had a lower level of daily functioning, and one-eighth had reduced working capacity, specifically because of headache.