Blood concentrations of cyclosporine A (CsA) >or=800 microg/l measured 2 h post-dosing, the C2 concentration, is necessary to obtain a maximal pharmacological effect and correlates well with transplant-related complications such as transplant rejection and toxicity. In an open crossover study CsA blood levels were measured during 24 h to generate a pharmacokinetic profile on days 1, 8 and 15 after starting CsA infusion in 21 haematopoietic allogeneic stem cell transplant recipients who were receiving intravenously CsA 3 mg/kg/day either by continuous infusion or by 2 h infusion given every 12 h. C2 levels after the 2 h infusion correlated better than C1 or C3 levels with the area under the concentration-time curve from 0 to 4 h (r2=0.62). C2 levels >or=800 microg/l were also achieved for 20 out of 24 (83%) of cases after the 2 h infusion of CsA without any increase of CsA-related toxicity but for only three of the 23 patients (13%) after continuous infusion. Therefore, we recommend CsA infusions in 2 h during transplant and perform C2 monitoring to obtain therapeutic C2 levels >or=800 microg/l.