The major adverse event associated with linezolid treatment is thrombocytopenia. This study investigated risk factors associated with the development of thrombocytopenia in patients who received linezolid therapy. This was a retrospective study of patients treated with linezolid 600 mg q12h, by the oral or parenteral route, between July 2007 and June 2009. Thrombocytopenia was defined as a ≥ 10 × 10⁴ cells/mm³ decrease from the baseline or a ≥ 30% reduction. Thirty-three variables were analyzed as risk factors associated with thrombocytopenia. Multivariate analysis was performed with SPSS version 16 software. This study included 331 patients (221 male, 58.0 ± 20.2 years). The mean duration of linezolid therapy was 10.5 ± 8.7 days. Thrombocytopenia occurred in 128 patients (38.7%), of whom 21 required platelet transfusion. Thrombocytopenia developed 7.4 ± 4.8 days after linezolid initiation, and the platelet count recovered 12.3 ± 7.8 days after withdrawal. In the final steps of stepwise logistic regression analysis, 5 variables were selected: duration of linezolid therapy ≥ 14 days [odds ratio (OR) 3.04, 95% confidence interval (CI) 1.73-5.34, P < 0.001], creatinine clearance < 50 mL/min (OR 2.32, 95% CI 1.45-3.74, P < 0.001), chronic liver disorders (OR 1.63, 95% CI 0.92-2.86, P = 0.092), and respiratory tract infections (OR 1.75, 95% CI 1.05-2.91, P = 0.031). In contrast to findings with parenteral administration, oral administration of linezolid decreased the risk of thrombocytopenia (OR 0.38, 95% CI 0.15-0.97, P = 0.042). The time to the onset of thrombocytopenia in patients with creatinine clearance <50 mL/min was significantly shorter than that in patients with creatinine clearance ≥ 50 mL/min (6.7 ± 4.4 and 8.5 ± 5.2 days, P = 0.039), and in patients administered orally the time to the onset of thrombocytopenia was 10.4 ± 6.6 days. Several factors influenced the occurrence of linezolid-induced thrombocytopenia. Dose adjustment may be required in at-risk patients, especially those with renal dysfunction.