May–Hegglin anomaly (MHA), Sebastian syndrome (SBS) and Fechtner syndrome (FTNS) are autosomal‐dominant macrothrombocytopenias with Döhle‐like leucocyte inclusions. These diseases are due to mutations of the MHY9 gene, encoding the heavy chain of non‐muscle myosin IIA (NMMHC‐A). We investigated the NMMHC‐A localization in blood cells from eight MHA, SBS or FTNS patients with known MYH9 mutations. All the patients showed an altered localization of NMMHC‐A in granulocytes and platelets, suggesting that Döhle‐like bodies are due to the aggregation of NMMHC‐A in the cytoplasm. Therefore, immunocytochemistry for NMMHC‐A is a simple and sensitive method to detect pathological phenotypes of granulocytes and platelets in the diagnosis of MYH9‐related disorders.