We studied the timing of a positive heparin-induced thrombocytopenia (HIT) test result relative to changes in platelet count and anticoagulant use. We obtained platelet counts in 100 consecutive HIT+ cases before, during, and after heparin therapy; 43 cases met study criteria and were included in study part 1. In part 2, platelet counts at the time of the HIT test in 2 groups (100 each HIT+ and HIT- cases) were compared. In part 1, cases could be divided into 4 groups based on the diagnosis of HIT relative to platelet counts (1, within 1-2 days of a major drop in platelet count [11.6%]; 2, after > 2 days of a major fall in platelet count [41.9%]; 3, in patients with already low platelet counts [27.9%]; 4, after platelet count was rising [18.6%]). In study part 2, the mean platelet counts for the HIT+ and HIT- groups were almost identical. HIT should be suspected in any thrombotic patient who had a previous decline in platelet count, has a low platelet count, or has a rising platelet count after a previous decline in association with heparin exposure. In study part 2, 1 platelet count value at the time of the HIT test did not provide useful information.