Thrombocytopenia is associated with increased patient risk. However, the costs of this complication are not well defined. This study assessed the impact of thrombocytopenia on in-hospital costs using results from CATCH, an observational study that examined 1988 consecutive patients receiving prolonged heparin therapy (> or =96 h). Thrombocytopenia was defined as: (group 1) an absolute reduction in platelet count to <150 x 10(9)/L; (group 2) a relative reduction in platelet count of >50% from admission levels; or (group 3) both criteria. We found that the development of thrombocytopenia was associated with significantly higher total in-hospital costs for all groups: (group 1) (difference, $8,222; 95% CI, $5,020-$11,425; P<.001); (group 2) (difference, $15,429; 95% CI, $7,472-$23,385; P<.001); and (group 3) (difference, $27,077; 95% CI, $22,901-$31,252; P<.001). However, in our adjusted model, longer lengths-of-stay and greater use of blood transfusions accounted for most incremental in-hospital cost differences.