Summary Three related patients are presented who show a congenital coagulation disorder with laboratory features intermediate between classical factor‐VII and factor‐X deficiencies. A woman and two men had suffered from bleeding since early childhood, with epistaxis, bleeding from the gums, post‐traumatic haemarthroses, bleeding after tooth extractions and other surgical procedures. Investigation demonstrated a prolonged prothrombin time, prolonged partial thromboplastin time, abnormal prothrombin consumption and abnormal thromboplastin generation corrected by normal serum. Platelet and vascular tests were normal and no hyperfibrinolysis was found. Factors I, II, V, VII, IX, XI and XII were within normal limits in all three patients. Mutual correction was demonstrated with a known factor‐VII‐deficient plasma but not with Stuart (X‐deficient) plasma. Factor‐X assay yielded low (4–9%) levels using tissue whole thromboplastin or tissue partial thromboplastin; but the results were normal with a Stypven‐cephalin mixture. In agreement with these results, the Stypven‐cephalin clotting time, the Stypven clotting time and the factor II + factor X level using a Stypven‐cephalin mixture were normal, ‘correction’ being attributable to the Russell's Viper venom. These results were thought to indicate an abnormal factor X rather than a real deficiency. The presence of abnormal factor X was demonstrated by the antibody neutralization technique and by the immunodiffusion studies. The defect, like classical factor X deficiency, is transmitted as an autosomal incompletely recessive trait. The heterozygote population has factor‐X levels varying from 32% to 55% of normal and are usually asymptomatic. The term ‘Factor X Friuli’ is proposed for the abnormality, due to a locally common mutant gene.