Patterns of C-reactive protein (CRP) release were derived from frequent CRP measurements in a cohort of 66 consecutive patients receiving allogeneic bone marrow transplants (BMT) in our unit. Based on a retrospective study of clinical events occurring within the first 40 days after BMT, patients with major transplant-related complications (MTC+ group, n = 22) could be separated from those with fever or mild complications only (MTC- group, n = 44). Treatment-related mortality in the MTC+ group was significantly higher: 32 vs 0% (P < 0.001). Major complications included veno-occlusive liver disease (VOD), severe endothelial leakage syndrome (ELS), pneumonitis and acute GVHD >II. The severity of complications was reflected by the patterns of CRP release with continuously high levels preceding the maximal signs and symptoms of MTC. Univariate analysis showed that, among other variables (sex, age, disease status at transplant, +/- TBI in the conditioning regimen, +/- use of myeloid growth factors after BMT, time to reach PN >200/mm3), three factors were significantly associated with MTC: maximal levels of CRP during the post-transplant episode (CRPmax) (296.6 +/- 91.8 vs 88.9 +/- 55.8 mg/100 ml, P < 0.001), the use of unmanipulated graft (no T depletion) (46.9 vs 12.5%, P < 0.009) and the CRP level on the day of BMT (CRPo) (42.7 +/- 55.4 vs 18.2 +/- 19.5, P = 0.045). In multivariate analysis, using a stepwise logistic regression model including the same variables, CRPmax appeared to be the strongest independent variable (P < 0.001) and a reliable (94% accuracy) parameter to assess the risk of MTC. Based on this model, CRP levels of 200 and 300 mg/100 ml are associated with a risk of 48 and 94% of developing MTC, respectively. We conclude that CRP monitoring after BMT identifies patients at risk of severe transplant-related complications and mortality.