Objective: To describe a novel missense mutation in the antithrombin gene associated with antithrombin deficiency type I in a 40-year-old man with retinal vein occlusion. Design: Investigational case report. Results: Ophthalmoscopy of the right eye showed hemicentral retinal vein occlusion. The patient’s medical history was negative for glaucoma or cardiovascular risk factors. Screening for thrombophilic disorders revealed antithrombin deficiency type I. Based on a genetic analysis, a novel missense mutation of a transition of guanosine to cytosine at nucleotide position 9840 was detected, predicting the replacement of aspartic acid by histidine encoded by codon 366 (D366H) in exon 5. Conclusions: Selective screening may be helpful in identifying patients with retinal vein occlusion with thrombophilic defects. When ordering laboratory tests in patients with retinal vein occlusion, antithrombin deficiency type I should be considered in the differential diagnosis. Clinical Relevance: Our results contribute to a better understanding of the molecular bases of antithrombin deficiency, adding a novel entry for the molecular defects causing antithrombin deficiency type I. Moreover, the identification of this thrombophilic disorder in retinal vein occlusion may be relevant to the issue of the initiation and duration of oral anticoagulant therapy. Objective: To describe a novel missense mutation in the antithrombin gene associated with antithrombin deficiency type I in a 40-year-old man with retinal vein occlusion. Design: Investigational case report. Results: Ophthalmoscopy of the right eye showed hemicentral retinal vein occlusion. The patient’s medical history was negative for glaucoma or cardiovascular risk factors. Screening for thrombophilic disorders revealed antithrombin deficiency type I. Based on a genetic analysis, a novel missense mutation of a transition of guanosine to cytosine at nucleotide position 9840 was detected, predicting the replacement of aspartic acid by histidine encoded by codon 366 (D366H) in exon 5. Conclusions: Selective screening may be helpful in identifying patients with retinal vein occlusion with thrombophilic defects. When ordering laboratory tests in patients with retinal vein occlusion, antithrombin deficiency type I should be considered in the differential diagnosis. Clinical Relevance: Our results contribute to a better understanding of the molecular bases of antithrombin deficiency, adding a novel entry for the molecular defects causing antithrombin deficiency type I. Moreover, the identification of this thrombophilic disorder in retinal vein occlusion may be relevant to the issue of the initiation and duration of oral anticoagulant therapy.