Increase of factor VIII activity (FVIII) after physical exercise has been reported in healthy subjects and small-scale studies in patients with coagulopathies. The aim was to study whether moderate and mild haemophilia A patients are able to increase their endogenous FVIII activity levels by physical activity. We studied changes in FVIII activity levels after high-intensity exercise in 15 haemophilia A patients, 20–39 years, eight with moderate, seven with mild haemophilia. Patients cycled until volitional exhaustion, blood samples were drawn before and 10 min after the exercise test. FVIII activity increased 2.5 times (range 1.8–7.0 times), for both severities. Absolute increases were markedly different: median 7 IU dL−1 (range 3–9 IU dL−1) in patients with moderate, compared to 15 IU dL−1 (range 6–62 IU dL−1) in mild haemophilia patients. VWF and VWFpp increased independently of severity; median 50% (range 8–123%) and median 165% (range 48–350%), respectively, reflecting acute release of VWF. These observations may be used to promote high-intensity activities before participating in sports for moderate and mild haemophilia A patients, to reduce bleeding risk. Further studies are warranted to fully appreciate the clinical significance of exercise on different levels of intensity in patients with mild and moderate haemophilia A.