Background: Immune-mediated thrombocytopenia (IMT) is a common hematologic disorder in dogs. Human intravenous immunoglobulin (hIVIG) may have a beneficial effect in canine IMT. Hypothesis: A single hIVIG infusion (0.5 g/kg) in dogs with presumed primary IMT (pIMT) is a safe adjunctive emergency treatment to accelerate platelet count recovery and shorten hospitalization time without increasing the cost of patient care. Animals: Eighteen client-owned dogs with a presumptive diagnosis of pIMT. Methods: Prospective, randomized, double-blinded, placebo-controlled clinical trial. Results: There were no identifiable immediate or delayed adverse reactions associated with hIVIG administration over a 6-month period. The median platelet count recovery time for the hIVIG group was 3.5 days (mean SD: 3.7 1.3 days; range, 2-7 days) and 7.5 days (mean SD: 7.8 3.9 days; range, 3-12 days) for the placebo group. The median duration of hospitalization for hIVIG group was 4 days (mean SD: 4.2 0.4 days; range, 2-8 days) and 8 days (mean SD: 8.3 0.6 days; range, 4-12 days) for the placebo group. There was no significant difference between groups with respect to expense of initial patient care, whereas significant reduction in platelet count recovery time (P 5.018) and duration of hospitalization (P 5.027) were detected in the hIVIG group. Conclusions and Clinical Importance: Compared with corticosteroids alone, adjunctive emergency therapy of a single hIVIG infusion was safe and associated with a significant reduction in platelet count recovery time and duration of hospitalization without increasing the expense of medical care in a small group of dogs with presumed pIMT.