Sticky platelet syndrome (SPS) was first described in 1983. However, not until 1995 did the prevalence of SPS receive significant recognition in the medical literature. In 1995 we began to routinely add an SPS evaluation to patients re ferred for assessment for causation of arterial and venous thromboses to a large thrombosis hemostasis referral center. The results of our first 2-year experience suggest SPS to be a common cause of arterial and venous thromboses. With respect to otherwise unexplained venous thrombosis, the prevalence of SPS approximates that of activated protein C resistance (APC- R). During the past 24 months, we have evaluated 153 patients referred for evaluation of unexplained arterial or venous events. An evaluation for common and uncommon blood coagulation protein defects and SPS has been applied to these patients. It has been found that SPS accounted for about 21 % of otherwise unexplained arterial events (acute myocardial infarction, cere brovascular thrombosis, transient cerebral ischemic attacks, retinal thrombosis, and peripheral arterial thrombosis) and ac counted for about 13.2% of otherwise unexplained venous events (deep vein thrombosis, with or without pulmonary em bolus). These findings strongly suggest SPS to be a common cause of arterial and venous thromboses and a workup for SPS should be considered a routine assay in the workup of indi viduals with otherwise unexplained arterial or venous throm botic events. Because treatment with heparin or warfarin will not alleviate the thrombotic tendency of SPS, but simple aspirin therapy almost always will correct the defect and protect the individual from second events, it is particularly important to define the presence of this defect. Key Words: Thrombosis— Platelet defects—DVT—Coronary thrombosis—Cerebral thrombosis—Recurrent miscarriage.