We have studied the kinetics of the N-terminal peptide of type III procollagen (NP3P) after BMT as a marker for the development of hepatic fibrosis in veno-occlusive disease (VOD). Four patients with clinically apparent VOD were retrospectively assayed and demonstrated a very high NP3P level. NP3P was also prospectively monitored at the beginning of conditioning and every week (8 patients) or every other day (14 patients) from the day of BMT (day 0) to day +28. Before conditioning the NP3P level (15.5 +/- 5.5 ng/ml) was twice normal and increased during the course of BMT in patients without VOD (21 ng/ml; range 6-35 ng/ml). In four patients who experienced VOD, the NP3P level exceeded 40 ng/ml by day 0 in two. The early rise of NP3P indicates that it is a valuable marker for the development of VOD before it becomes clinically apparent. These data suggest that VOD develops during preparation for BMT and that prophylaxis should therefore be started at this time.