Type I antithrombin deficiency is an autosomal dominant disorder associated with thromboembolic complications mainly related to single-point mutations in SERPINC1, the gene encoding antithrombin. Chromosomal rearrangements have been found in up to 10% of cases with type I antithrombin deficiency. We report here the first heterozygous deletion of SERPINC1 exon 1 identified in a 44-year-old man with type I deficiency who developed deep vein thrombosis of the left leg complicated by pulmonary embolism. This study demonstrates that the search for large gene rearrangements in SERPINC1 can be a useful diagnostic approach, particularly in patients with type I antithrombin deficiency without mutations in SERPINC1.