Veno-occlusive disease of the liver (VOD) is a life-threatening complication occurring in patients undergoing bone marrow transplantation (BMT). Although clinical signs and laboratory parameters such as elevation of serum bilirubin often suggest this condition, it would be useful to identify early biochemical markers for VOD. Fibrous alterations in the hepatic venules and small lobular veins occur during development of VOD; these changes are accompanied by the deposition of types I and III collagen in the liver tissue. Since the N- terminal propeptide of type III procollagen (PIIINP) is a sensitive marker of liver and lung fibrosis, we undertook a study to evaluate the usefulness of measurements of serum PIIINP in children with VOD. Seven of the 28 children who underwent BMT, both allogenic and autologous, developed VOD. All seven had an increase of more than 100 ng/mL in the serum PIIINP level, whereas only one of the remaining 21 children not affected by VOD had an increment of PIIINP more than 100 ng/mL (P = .0001). The levels of serum PIIINP were higher in the VOD group during the follow-up period of up to 91 days after BMT. The elevation of PIIINP also occurred at a stage of the disease usually preceding any other laboratory or clinical signs of VOD. Serum concentration of PIIINP thus seems to be of value as an early marker for VOD in children undergoing BMT.