OBJECTIVES: To estimate the incremental hospital costs and related length of stay associated with the development of thrombocytopenia in a general medical/surgical population. METHODS: We included patients from the Complications After Thrombocytopenia Caused by Heparin (CATCH) Registry treated with heparin for ³ 96 hours (n=1997). Hospital costs for all CATCH patients were estimated using hospital cost accounting system results for CATCH Registry patients enrolled at Duke University Medical Center (n=336). After adjusting for baseline characteristics, and procedures and complications occurring before 96 hours, we estimated incremental hospital costs associated with thrombocytopenia defined prospectively by a platelet count drop to < 150 x 103/mm3, a platelet count reduction > 50% from admission level, or both. RESULTS: Overall, 28% of CATCH patients receiving heparin for ³ 96 hours developed thrombocytopenia. These patients more commonly had a history of MI (23% vs. 15%), heart failure (29% vs. 24%), and experienced more in-hospital death (5.1% vs. 1.1%) than patients who did not develop thrombocytopenia. The total population's average length of stay was 14.5 days with mean hospital costs of $29,523, but both values were significantly greater among patients developing thrombocytopenia (18.0 vs. 13.2 days, and $45,192 vs. $23,527). After adjustment, the development of thrombocytopenia in all groups remained a significant marker for increased hospital costs over patients without thrombocytopenia (incremental costs = $5262 in patients with a platelet count drop to < 150 x 103/mm3, $9340 in patients with a platelet count reduction > 50% from admission level, and $18,488 in patients meeting both criteria). CONCLUSIONS: The incremental economic impact of thrombocytopenia, experienced commonly among hospitalized patients, is substantial. Strategies that minimize or effectively prevent the development of thrombocytopenia may therefore be economically attractive.