Primary fibromyalgia (PFM) and the irritable bowel syndrome (IBS) are both common conditions which account for 30% or more of referrals to rheumatology and gastroenterology clinics. An association between symptoms in PFM and IBS has been suggested but the frequency with which they coexist has not been assessed. The aim of this study was to examine the prevalence of each condition in groups of patients with PFM and IBS compared to normal and disease control populations. We studied four patient groups, 20 patients in each group, with PFM, IBS, inflammatory arthritis, inflammatory bowel disease and also 20 normal controls. Using strict diagnostic criteria, each group was assessed by two investigators for symptoms and signs of PFM and IBS. Sigmoidoscopy was performed when indicated. Results indicate that 70% (14/20) of the PFM patients had IBS and 65% (13/20) of the IBS patients had PFM. This compared with the control groups where 12% (7/60) and 10% (6/60) had PFM and IBS respectively. In conclusion, these results indicate that PFM and IBS frequently coexist. A common pathogenetic mechanism for both conditions is therefore suggested.