The range of tests used in the evaluation of thrombophilia has been altered by the recent recognition of common genetic defects predisposing to thrombosis such as factor VLeiden (FVR506Q), enzyme deficiencies causing hyperhomocysteinemia, and improvement in the sensitivity and utilization of assays for antiphospholipid antibodies. In this study, the outcomes of laboratory evaluation of 402 patients with thrombophilia were reviewed and correlated with clinical data. A predisposing factor was present (positive diagnosis, group A) in 110 patients (27%), the test results of 111 patients (28%) could not be definitively interpreted (equivocal results, group B), and the test results of 181 (45%) were normal (group C). The median age of the group A patients was 48 years (range, 3.7–88 years), suggesting that evaluation of patients over the age of 50 is worthwhile. Of the 110 patients in group A, 84% had single defects and 16% had combined defects. The most common defect was factor VLeiden (44 patients). Equal numbers of patients presenting with arterial and venous thromboses were evaluated. Patients with arterial events were less likely to have a definable laboratory defect (33 of 132 [25%]) than were those with venous events (50 of 136 [37%]). Factor VLeiden was the most frequent finding in patients with venous events, and lupus anticoagulant or anticardiolipin antibodies were the most frequent findings in patients with arterial events. Positive diagnoses were made in patients on anticoagulants, indicating that this should not preclude investigation. Our study confirms the need for thorough evaluation to assess thrombotic risk, and it reflects the impact of newly identified thrombophilic disorders on the expected outcome of laboratory evaluation for thrombophilia.