Acne fulminans is a rare, severe type of acne with unknown etiology. Ulcerative acne lesions, fever and musculoskeletal pain are typical symptoms. In addition, osteolytic or even destructive osteomyelitis-like bone lesions occur in many patients with acne fulminans. In the present study the degradation product (ICTP) of type I collagen, the most abundant collagen of the skeleton, was measured from the sera of patients suffering from acne fulminans. In 3 of 4 acne fulminans patients with active disease, the ICTP concentrations were clearly higher than the range of concentrations in age-matched controls. The mean concentration of ICTP in the acne fulminans patients was 17.6 +/- 6.0 micrograms/I, whereas the corresponding concentration in 6 patients with severe nodular acne was 6.9 +/- 2.1 micrograms/I. Increased uptake of radionuclide in bone scans was observed in all of the 4 patients with acne fulminans. The present results suggest that ICTP is increased in acne fulminans, due to the destruction of bone collagen matrix. ICTP could thus be used for monitoring the activity of acne fulminans affecting the skeleton.