Interferon-alpha (IFN-alpha) is used in the treatment of many haematological diseases and it is known that IFN-alpha may affect bone turnover. The effect of IFN-alpha on bone metabolism was studied in 10 haematological patients. The mean duration of the treatment was 4 (range: 2.8-7.2) months. Besides the usual markers of bone metabolism, levels of the cross-linked C-terminal telopeptide of type I collagen (ICTP), the N-terminal propeptide of type I procollagen (PINP) and the bone-specific alkaline phosphatase were measured. The bone mineral density was measured by computed tomography. During IFN-alpha treatment, serum ICTP decreased from a mean of 5.4 (range: 1.8-12.4) to 3.6 (range: 1.4-8.8) microg/l (P = 0.017). All other variables reflecting bone metabolism remained unaltered during IFN-alpha treatment. The bone mineral density remained unchanged. It was concluded that the observed decrease in ICTP may be an indicator of a beneficial therapeutic effect of IFN-alpha on bone turnover, resulting in decreased bone resorption. However, it is possible that elevated pretreatment ICTP values reflected disease of the bone marrow.