Cardiovascular fitness training has been suggested as a treatment for the fibrositis/fibromyalgia syndrome. Thirty-four patients with fibrositis/fibromyalgia who met Smythe's original criteria were randomly assigned to enter either a cardiovascular fitness training program or a program consisting only of flexibility exercises. Patients met in supervised groups three times weekly for a 20-week observation period. The cardiovascular fitness group underwent gradual heart rate-elevated training using a bicycle ergometer and achieved a 29.1 +/- 24.4 percent increase in peak work capacity at 170 beats per minute (PWC-170). Patients undergoing flexibility training had a net reduction in their PWC-170 scores of 4.3 +/- 9.4 percent. Patients in the cardiovascular fitness group had statistically significant improvements in the visual analogue pain scale of current pain intensity; total myalgic scores in which pain thresholds at five fibrositic tender points selected for acceptable intra-rater and inter-rater reliability were measured using a dolorimeter; percentage total body area affected as measured by self-administered pain diagrams; and patients' and physicians' global assessment scores. Psychologic profiles as measured by Symptom Checklist-90R also improved in the cardiovascular fitness group compared with the flexibility training group. It is concluded that cardiovascular fitness training is feasible in patients with fibrositis/fibromyalgia and that such training improves subjective measurements of pain-reporting behavior. A theoretic basis for improvements in pain measurement scales and psychologic profiles is discussed in light of recent literature.