INTRODUCTION: A reduced platelet count (PLT) is a frequent post-operative finding in orthopedic surgery patients. Despite its prevalence, the characteristics of post-surgical thrombocytopenia have not been well described. METHODS: A retrospective chart review was conducted on patients who underwent a knee or hip replacement from 2012 to 2015. Patients who received heparin were excluded. RESULTS: A total of 56 patients were analyzed on post-operative days 0 to 4. By day 1, 90.9% of the patients experienced a reduction in their platelet counts. The lowest mean platelet count (nadir) occurred on day 2 (201.3 × 109/L). The average decrease in the platelet count from the baseline was 24% (95%CI: 20.6 - 27.2). The change in the platelet count from the baseline ranged from a 49.6% drop to a 14.2% increase. A substantial portion of patients experienced thrombocytopenia, with 28% occurring on day 2. Platelet counts less than 100 × 109/L occurred only once. The percent decrease in the platelet count from the baseline to any other time point was significantly larger in patients aged > 65 years, compared to patients aged ≤ 65 years (p = 0.007). Specifically, the average drop in the platelet count at the nadir (day 2) relative to the baseline was 27.8% in patients aged > 65 years, compared to 19.5% in patients aged ≤ 65 years. CONCLUSIONS: A reduction in the platelet count is a frequent post-operative finding in orthopedic surgery patients, even after removing confounding factors, such as heparin exposure, but clinical thrombocytopenia is uncommon. Alternative etiologies should be considered when the platelet count is less than 100 × 109/L. Vigilance should also be considered regarding elderly patients.