Summary. The indications for and the efficacy of prophylaxis in adults with haemophilia remain controversial. It is unclear whether the benefits of secondary prophylaxis outweigh the costs, because adults with haemophilia usually already have established arthropathy. The objectives of secondary prophylaxis in this group are therefore more limited than the objectives of primary prophylaxis in children. It is also uncertain whether primary prophylaxis should stop or continue once adulthood is reached. Some individuals with near-normal joints may stop prophylaxis in early adulthood and then bleed infrequently. Others who stop prophylaxis begin to bleed frequently and suffer progressive arthropathy; these patients should probably have continued prophylaxis. There is no satisfactory method for selecting patients for continued prophylaxis. Adult prophylaxis is less well studied than prophylaxis in children. A few studies with a small number of patients suggest that adults treated with prophylaxis experienced fewer bleeding episodes, less pain and improved quality of life compared with those treated on demand. The mean annual cost of prophylaxis tends be substantially higher for adults than for children, largely owing to the high cost of clotting factor. Here we review the literature regarding the prophylactic treatment of adult patients with haemophilia A, including studies of the discontinuation of prophylaxis. These studies and others all show clinical benefit from prophylaxis in adulthood and suggest the possibility that optimized prophylaxis (e.g. tailoring an intermediate- or low-dose regimen in patients who bleed infrequently) may improve clinical outcome. The cost-effect argument is more difficult to sustain in adults compared with children; however, the cost of prophylaxis may be counterbalanced by indirect factors, such as days gained at work, reduced hospitalizations, reduced need for orthopaedic surgery and improved quality of life.