The usefulness of a calculator programme that enables individualization of the dosage of Factor VIII, based on the concentration-time data measured after a test-dose, was assessed. The programme was tested in 12 haemophiliacs who required multiple-dose treatment with Factor VIII. Individual kinetic parameters were estimated in each patient from the plasma level data following the test-dose. Then, each patient received the multiple dose treatment with Factor VIII according to the dosage regimen suggested by the calculator programme. The time-curve of Factor VIII plasma levels at steady state was predicted by using the kinetic variables previously estimated. The plasma levels of Factor VIII were measured in all patients at steady state. As a result, good agreement between predicted and measured steady state concentrations was observed (r2 = 0.9797; P less than 0.001). The calculator programme tested in the present study appears to be a useful tool for individualizing the dosage regimen of Factor VIII in haemophiliacs.