Forty-two patients with primary fibromyalgia were randomized into a 20-week program consisting of either cardiovascular fitness (CVR) training or simple flexibility exercises (FLEX) that did not lead to enhanced cardiovascular fitness. Patients were supervised by the same medical fitness instructors. Patients in neither group had contact with members of the other group, and were blinded as to the exercise taught to the alternative group. Groups met for 60 minutes 3 times each week. The compliance rate was 90%. Thirty-eight patients completed the study (18 with CVR training and 20 with FLEX). Blind assessments (standardized in preliminary trials to achieve acceptable inter-rater agreement) were performed by the same 2 examiners. After 20 weeks, patients receiving CVR training showed significantly improved cardiovascular fitness scores compared with those receiving FLEX training (t[35] = -4.22, P less than 0.003). Logistic regression analysis showed clinically and statistically significant improvements in pain threshold scores, which were measured directly over fibrositic tender points, in patients undergoing CVR (t[35] = 2.21, P less than 0.04). There was also a trend toward improvement in pain scores (visual analog scale) in the CVR group, but this did not reach statistical significance. There was no improvement in the percentage of body area affected by fibrositic symptoms or the number of nights per week or hours per night of disturbed sleep (self-report inventories). However, compared with the FLEX group, the CVR-trained patients improved significantly in both patient and physician global assessment scores.(ABSTRACT TRUNCATED AT 250 WORDS)