The emergence of a population of relatively healthy adults with severe haemophilia A presents a unique challenge for haemophilia care in the 21st century. Understanding how best to continue, restart, initiate or modify prophylaxis in younger and older adult patients is essential to optimizing their care. To elucidate practice and outcome data, a survey was sent to 23 US hemophilia treatment centers (HTCs); 10 centers responded, providing data concerning up to 145 adults (mean age of 34 years). Forty-eight patients (33%) were on regular prophylaxis when first seen at the HTC; the prophylactic regimen was modified for 22/48 (46%), often because of breakthrough bleeding. Five of 21 patients (24%) for whom data were available discontinued prophylaxis. Three of those five patients (60%) experienced increased bleeding episodes and the other two (40%) subsequently resumed regular prophylactic infusions because of the increased bleeding. Of the 77 patients not initially receiving prophylaxis for whom data were available, prophylaxis was started or resumed in all. The prophylactic regimen was modified in 57/77 patients (74%) at some point during treatment, often because of breakthrough bleeding. Of the 55 patients whose prophylactic regimens were modified for whom data were available, 22 (40%) discontinued prophylaxis. Thirteen of 20 patients (65%) for whom data were available experienced an increase in bleeding episodes and 7/18 patients (39%) who had discontinued prophylaxis and for whom data were available subsequently resumed regular prophylactic infusions because of bleeding. These findings suggest that prophylaxis prevents bleeding in adults with severe haemophilia A and that discontinuation of the prophylactic regimen is associated with increased bleeding events.