The aim of this study was to define factors that significantly influence the early kinetics of donor chimerism after transplantation. In a retrospective study, the percentage of donor chimerism in peripheral blood measured with sex-chromosome-specific probes and fluorescence-in situ hybridization was analyzed in 184 recipients of allogeneic hematopoietic cells between days 1 and 30. Using a generalized linear model for longitudinal observations, the dose of CD34+ cells infused had a significant impact on the slope of donor chimerism. In multivariate analysis, cell doses of 2-8 x 10(6)/kg (P=0.001) and <2 x 10(6) CD34+ cells/kg (P<0.0001) were associated with slower increase of donor chimerism compared to >8.0 x 10(6) CD34+ cells/kg. In addition, fludarabine-based reduced-intensity conditioning resulted in a significant delay of donor cell increase compared to standard conditioning therapy (P=0.0001). The application of chemotherapy before the start of conditioning (P=0.0003) and the use of antithymocyte globulin (P=0.003) were associated with a faster increase of donor chimerism. The factors identified in this study can be used to predict the kinetics of early donor chimerism for an individual patient.