Regular prophylaxis and the cumulative incidence of inhibitor development. The relative risk for regular prophylaxis was 1.0 in the period from 1 to 10 exposure days, 0.95 from 11 to 20 exposure days, 0.22 from 21 to 30 exposure days, 0.27 from 31 to 40 exposure days, and 0.32 from 41 to 75 exposure days.