The recent literature contains reports of thrombotic episodes occurring in patients with human immunodeficiency virus (HIV) infection and various abnormalities predisposing to a hypercoagulable state have also been reported in such patients. To study the incidence of thrombosis in patients infected with HIV, and to assess the correlation of thrombosis with the degree of immunosuppression as well as the association with active illnesses and neoplasms, we reviewed the charts of 131 patients, which include all the patients with the diagnosis of HIV admitted or seen in the clinic between January 1, 1993, and January 1, 1998. The diagnosis of thrombosis was based on documented reports of venous plethysmography or venography for deep venous thrombosis and ventilation-perfusion scan or pulmonary angiography for pulmonary embolus. Risk factors for thrombotic disease were evaluated including general risk factors such as family history, ambulatory status, medications, and data were also collected regarding CD4 cell counts and the presence of concurrent or remote opportunistic infections, acquired immune deficiency syndrome (AIDS)-related malignancy or other AIDS-related diseases at the time of diagnosis of the thrombotic event. We also reviewed the medical literature via MEDLINE and found 45 cases of patients with HIV who developed thromboembolic complications. We found thrombotic complications in 9 of 37 patients with a CD4 count less than 200 cells/mm3 and in 1 of the remaining 94 patients with a CD4 count more than 200 cells/mm3. The difference was significant, with p = 0.00004, and the estimated odds of an event given CD4 cell counts less than 200/mm3 is 29.89 (95% confidence interval). Three patients had abnormalities of anticoagulation proteins. There was a history of opportunistic infections in 5 patients and malignancy in 3 patients. Two patients with autoimmune hemolytic anemia (AIHA) secondary to HIV-infection developed PE upon transfusion of packed red blood cells. The results of this study suggests that AIDS appears to predispose to thrombosis. It also revealed a significant correlation between thrombotic disease and CD4 counts (<200/mm3) as well as the presence of opportunistic infections, AIDS-related neoplasms, or autoimmune disorders associated with HIV such as AIHA. Therefore, clinicians caring for these patients should be aware of thromboembolic disease as a possible complication of AIDS. Further studies to elucidate the mechanisms underlying this abnormal hemostatic profile, the epidemiology, and to answer several questions such as should patients with risk factors for HIV infection who develop thromboembolic complications be further evaluated including tests for HIV are warranted.