It has been suggested that ultrapure clotting factor products carry a greater risk for inhibitor development in patients with haemophilia. We compared the incidence of inhibitors in 59 previously untreated patients (PUPs) with severe haemophilia (endogenous factor VIII < 0.01 U L-1) A, who were initially treated with cryoprecipitate or intermediate purified products, with that in 22 patients exclusively treated with monoclonally purified and recombinant factor VIII. Persistent inhibitors were those with 1 Bethesda unit per mL or more, on more than one occasion, combined with a decrease in recovery. Incidences of persistent inhibitors were 17% (10/59) for patients who were treated with cryoprecipitate or intermediate-purity products and 9% (2/10) for monoclonally purified and recombinant factor VIII. Transient inhibitors appeared to develop earlier during treatment with ultrapure products as compared to treatment with intermediate/low-purity products. In conclusion, ultrapure products appear not to carry a higher risk for inhibitor development.