Serum concentrations of procollagen type III aminoterminal peptide have previously been reported to be low in some patients with primary fibromyalgia and the aim of this study was to determine if such patients differ clinically from primary fibromyalgia patients with normal levels of procollagen type III aminoterminal peptide. Subjective symptoms, tender points and dynamic muscle strength in 45 women with primary fibromyalgia were related to serum concentrations of procollagen type III aminoterminal peptide. Patients with low serum concentrations of procollagen type III aminoterminal peptide had more symptoms, a higher frequency of tender points and lesser quality of sleep compared to patients with normal serum concentrations of procollagen type III aminoterminal peptide (P less than 0.05). They also had a lower dynamic muscle strength (P less than 0.0005). We conclude that the serum concentrations of procollagen type III aminoterminal peptide of primary fibromyalgia patients are connected to the disease impact.