While primary prophylaxis is a well-established and recommended method of care delivery for children with severe haemophilia, fewer studies have documented the benefits of secondary prophylaxis started in adolescence or adulthood. To evaluate the role of secondary prophylaxis started in adolescent and adult severe haemophiliacs, a retrospective observational cohort study was conducted in 10 Italian Centres that investigated 84 haemophiliacs who had bled frequently and had thus switched from on-demand to prophylactic treatment during adolescence (n = 30) or adulthood (n = 54). The consumption of clotting factor concentrates, the orthopaedic and radiological scores, quality of life and disease-related morbidity were compared before and after starting secondary prophylaxis. Prophylaxis reduced the mean annual number of total and joint bleeds (35.8 vs. 4.2 and 32.4 vs. 3.3; P < 0.01) and of days lost from work/school (34.6 vs. 3.0, P < 0.01). A statistically significant reduction in the orthopaedic score was observed during prophylaxis in adolescents, but not in the whole cohort. Patients used more factor concentrates with corresponding higher costs on prophylaxis, but experienced a better quality of life. With respect to on-demand treatment, higher factor consumption and cost of secondary prophylaxis were balanced by marked clinical benefits and greater well-being in this cohort of adolescent/adult haemophiliacs.