Type 1 antithrombin III (ATIII) deficiency, which is the commonest form of inherited ATIII defect, is characterized by a quantitative reduction in both immunologically and functionally detectable protein. This condition is associated with a high incidence of thromboembolic disorder. Previous investigations have shown that the ATIII genes in the majority of cases are grossly intact, but the precise underlying molecular defects remain unknown. We have investigated the molecular basis of a type 1 ATIII deficiency in an Italian kindred by enzymatic amplification of the ATIII gene sequences in affected family members and direct sequencing of the amplified genomic DNA. A novel mutation, the deletion of a single T in the second position of codon 119, was identified in each of the affected individuals. The resulting frameshift leads to a premature termination in codon 126, effectively resulting in a null allele.