The purpose of this study was to assess the incidence, pattern and severity of somnolence and fatigue in patients treated with cranial irradiation for primary brain tumours and to identify factors that may influence or mediate symptoms. A detailed prospective study was carried out of 19 patients who received high-dose (45-55 Gy) cranial irradiation as treatment for primary brain tumours. Data were collected for each patient over a 3 month period using a prospective diary utilizing visual analogue scales of common somnolence symptoms and fatigue, and detailed interviews at 2, 6 and 12 weeks following the completion of treatment. Sixteen patients developed somnolence syndrome following treatment. Time series analysis identified a cyclical pattern to the symptoms, with a period of drowsiness and fatigue occurring from day 11 to day 21 and from day 31 to day 35 after radiotherapy. The principal symptoms were those of excessive drowsiness, feeling clumsy, an inability to concentrate, lethargy, being mentally slow and fatigue. Patients treated with accelerated (n = 11) compared with more conventional (n = 8) fractionation experienced more severe drowsiness and fatigue (P < 0.01), although there was no difference in the pattern or the incidence of symptoms. Interview data suggested that patients frequently attributed their symptoms of somnolence to 'flu or other ailments. The unexplained and overwhelming nature of the symptoms was a cause of anxiety. The prospective assessment of symptoms following radiotherapy highlighted a more detailed definition of the symptom complex and pattern of occurrence. Somnolence syndrome is a collection of symptoms consisting of drowsiness, lethargy and fatigue. Forewarning patients and planning supportive management around times of drowsiness and fatigue can help to reduce the anxiety that these symptoms cause.