The value of serum procollagen peptide (PICP) as a non-invasive index of bone formation was studied in 18 patients established on haemodialysis. There was a significant correlation between PICP and serum alkaline phosphatase activity (ALP; r = 0.55, P < 0.05), and between PICP and osteocalcin (r = 0.53, P < 0.05). PICP also correlated significantly with histomorphometric indices of bone formation, particularly bone formation rates (BFR) as estimated by the tetracycline double-labelled technique (r = 0.74, P < 0.01), but not with those of bone resorption. There was a similar relationship between BFR and ALP. From the regression analyses, a normal BFR was associated with normal PICP values despite the absence of renal function, suggesting that the impact of renal function on serum concentrations of PICP may not be large. Seven patients had histochemical evidence for significant aluminum overload. In these patients the expected suppression in biochemical and histological indices of bone formation was associated with inappropriately raised PICP concentrations. The mechanism of this discrepancy is not clear, but caution is advocated in the interpretation of PICP in the presence of significant aluminium overload. Our findings otherwise suggest that PICP may be a useful non-invasive index of bone formation in patients on haemodialysis.