The study was carried out to estimate the prevalence of chronic fatigue syndrome (CFS) in Iceland. No previous prevalence studies known to us have been undertaken in Iceland or in Scandinavia. A 95-item custom-made questionnaire was sent to 4000 randomly selected people. The response rate was 63%. The questionnaire was constructed to include questions on all the items found in the four most common criteria for diagnosing CFS; the criteria being Australian, British and American. Results show very different prevalences according to the criteria used. The prevalence ranged from 0 to 4.9%, with the most established criteria yielding a prevalence of 1.4%. Re-test validity of the questionnaire was good, the following results are based on the selection criteria by Fukuda et al. (Fukuda K, Straus SE, Hickie I, Sharpe MC, Dobbins JG, Komaroff A, et al. The chronic fatigue syndrome: a comprehensive approach to its definition and study. Ann Int Med 1994;121:953-9). Women were in a majority (78%); their mean age was 44, they were fully employed and worked long hours. They believed that the onset of their symptoms was stress related. The type of work was unskilled in the majority of cases. A significant proportion of the males felt a constant buzzing in their ears (P < 0.05). Food suppliants were used daily by significantly more women than men (P < 0.01). Men had more frequently phobic symptoms (P < 0.001) than did women. Differences were found in the prevalence of phobia and panic (P < 0.001) between women in the CFS group compared to healthy ones. A positive correlation was found in the prevalence of phobia between women in the CFS group and those with Iceland Disease.