The eradication of inhibitory antibodies in patients with haemophilia A can be accomplished by frequent administration of high or intermediate doses of factor VIII (FVIII), so-called immune tolerance induction (ITI). This study monitored the distribution of IgG subclasses of anti-FVIII antibodies during ITI. FVIII-specific antibodies of subclass IgG1 were detected in all inhibitor patients tested, anti-FVIII IgG4 in 16, IgG2 in 10 and IgG3 in one of 20 patients analysed. Levels of anti-FVIII IgG1 and IgG4 correlated well with inhibitor titres as measured by Bethesda assay. In low-titre inhibitor patients, anti-FVIII antibodies consisted primarily of subclass IgG1 whereas, anti-FVIII antibodies of subclass IgG4 were more prominent in patients with high titre inhibitors who needed prolonged treatment or who failed ITI. Longitudinal analysis of 14 patients undergoing ITI revealed that the relative contribution of IgG subclasses was constant for most of the patients analysed. In two patients, the relative contribution of IgG4 increased during ITI. Overall, our findings document the distribution and dynamics of anti-FVIII IgG subclasses during ITI. Future studies will need to address whether monitoring the relative contribution of anti-FVIII subclasses IgG1 and IgG4 may be useful for the identification of patients who are at risk of failing ITI.