A number of observations suggest that severe factor IX deficiency (<1%) may be less clinically severe than the corresponding factor VIII deficiency: (i) Less factor consumption. There is evidence that patients with haemophilia B (HB) consume yearly less FIX for replacement therapy than patients with haemophilia (HA). Patient registries and data from various sources indicate that regular prophylaxis is implemented less frequently in HB. (ii) Less severe gene mutations. At variance with HA, missense gene mutations are prevalent in severe HB, supporting the view that some FIX may be produced in these patients, albeit not measurable in patient plasma by means of the relatively insensitive available assays. (iii) Less severe clinical symptoms. In the frame of a process meant to develop a score to express the varied clinical severity of both haemophilias in different patients, those with severe HB were less clinically severe, and hence had lower scores, than those with HA. (iv) Less need for orthopaedic surgery. Patients with severe HB needed joint arthroplasty (an indirect index of arthropathy severity) less frequently than those with HA, and this difference was maintained when various confounders were accounted for. In conclusion, these and other data give a hint that HB may be less clinically severe than HA. However, these data are not conclusive, because there are also fewer data in favour of a similar degree of severity of HA and HB.