Diabetic foot ulcer is a common complication in diabetic patients and the outcome of foot ulcer depends on the severity of ulcer and combined infection such as osteomyelitis. So antibiotics targeting Gram positive cocci including Staphylococcus aureus have been used frequently. However, during the antibiotic therapy, diverse adverse reactions could be happened including red man syndrome, chest pain, hypotension, thrombocytopenia, neutropenia and drug eruption. Among these reactions, hematologic adverse events such as pancytopenia are rare and it could be lethal if happened. This article reports a case of pancytopenia associated with vancomycin in treating the diabetic patient with infected foot ulcer.