A young female Somali cat was referred for investigation of chronic intermittent haematuria. Petechiae were found on the ears and ventral abdomen and further investigation revealed severe thrombocytopenia and megakaryocyte hyperplasia. Direct marrow immunohistochemistry detected anti-megakaryocyte autoantibody (immunoglobulin G), but extensive investigation failed to find secondary causes of immune-mediated thrombocytopenia, so a diagnosis of primary (autoimmune) immune-mediated thrombocytopenia was concluded. Thrombocytopenia persisted despite aggressive immunosuppressive therapy (prednisolone, azathioprine and vincristine) but resolved after oral prednisolone was replaced with dexamethasone.