An inherited antithrombin deficiency is an autosomal dominant thrombotic disorder. We identified two pedigrees of inherited type I antithrombin deficiency and two responsible mutations in each. A novel 21-22delAA appeared to have caused a frameshift with a premature termination at amino acid +63 in one patient and a large deletion including all seven exons was identified by multiplex ligation-dependent probe amplification in the other. Some asymptomatic relatives of the second patient had the same mutation. The present findings support the value of using more than one method of gene analysis and of studying the families of probands with inherited thrombotic disorders.