We investigated the prevalence of dyspnoea, according to a 5-grade score proposed by the World Health Organization (WHO), among 87 consecutive women (age 44.0 +/- 8.4, range 26-65 years), with severe chronic primary fibromyalgia (CPF). In total 73 women (84%) were dyspnoeic, 47, 17, six and four patients reported dyspnoea corresponding to WHO grades 1, 2, 3 and 4, respectively. In multivariate analysis WHO dyspnoea grade was the most important determinator of exercise capacity. The higher WHO dyspnoea grade reported, the lower exercise capacity and heart rate were reached at graded bicycle exercise tests (r = -0.48, P less than 0.001 and r = -0.40, P less than 0.001). WHO dyspnoea grade correlated directly to breathing frequency at rest (r = 0.45, P less than 0.001) and inversely to maximum inspiratory pressure (r = -0.29, P less than 0.01). It did not correlate with maximum expiratory pressure, nor with spirometric measurements of pulmonary function. It is concluded that dyspnoea is common among CPF patients and is not explained by cardiac or pulmonary causes, but may partly be due to diaphragmatic muscular insufficiency and physical inactivity.