The authors report the case of an active 32 year old man who developed right leg DVT. Before heparinisation, he was discovered to have a low antithrombin III level (biological activity (B) 60%, immunological level (I) 50) and a further inquiry showed the same abnormality in 4 members of the family, leading to a diagnosis of a congenital deficit: a 35 year old sister with a bilateral post-DVT changes had antithrombin III levels of 70% (B) and 45% (I); two nephews, sons of the affected sister: the one aged 5 years was asymptomatic despite antithrombin III levels of 50% (I) and 70% (B); the other had experience DVT at the age of 2 and, on oral anti-vitamin K drugs, had antithrombin III levels of 55% (I) and 67% (B) at the age of 15 years; the patient's brother died at the age of 29 of cerebral vein thrombosis after pulmonary embolism. The recurrence of local signs of DVT after 12 day's heparin therapy with AT III levels (B) of 40%, led to a change in management with infusion of purified AT III concentrate at a dose of 40 U per kg (2 500 U per hour). This induced a rise in AT III activity to over 100% and enabled early introduction of anti-vitamin K therapy. The patient remains asymptomatic after 6 months follow-up. This case illustrates the value of determining AT III activity in all patients who developed DVT without obvious reason.(ABSTRACT TRUNCATED AT 250 WORDS)