We set out to determine the relation between a general practitioner (GP) diagnosis of viral illness and development of chronic fatigue 6 months later. 618 subjects who attended GPs clinics in London, south, and southwest England and who received a diagnosis of viral illness were followed prospectively and fatigue was assessed by questionnaire after 6 months. At presentation, GPs recorded fatigue in 62.6% of subjects, usually since the onset of symptoms. 502 (81.2%) subjects completed the 6-month questionnaire, of whom 88 (17.5%) met criteria for chronic fatigue and 65 (12.9%) had no reported fatigue before the viral illness. Compared with a similar group of non-postviral GP attenders, the risk ratio for chronic fatigue in the present cohort was 1.45 (95% CI 1.14-2.04). Infective symptoms did not predict fatigue 6 months later. Psychiatric morbidity, belief in vulnerability to viruses, and attributional style at initial presentation were all associated with self-designated postviral fatigue. Logistic regression showed that somatic attributional style, less definite diagnosis by the GP, and sick certification were the only significant predictors of chronic fatigue after viral infection when other factors were controlled for. Chronic severe fatigue 6 months after GP-diagnosed viral illness is related to symptom-attributional style and doctor behaviour, rather than to features of the viral illness. Some subjects with apparent postviral fatigue had complained of tiredness before their presentation with a viral illness.