A new entity, namely early/prefibrotic primary myelofibrosis (PMF), was introduced as a subtype of PMF in the 2016 revised World Health Organization (WHO) criteria for myeloproliferative neoplasms (MPN). It was diagnosed based on histopathological features of bone marrow (BM) biopsy specimens together with clinical parameters leukocytosis, anemia, elevated lactate dehydrogenase (LDH) values, and splenomegaly. The aim of this study was to evaluate the prevalence of early/prefibrotic PMF in patients who were previously diagnosed with ET, and to compare clinical features at diagnosis and outcomes between early/prefibrotic PMF and essential thrombocythemia (ET) patients. BM biopsy samples obtained at the time of ET diagnosis were available in 42 patients. Sample reevaluation according to the 2016 revised WHO criteria revealed that early/prefibrotic PMF accounted for 14% of patients who were previously diagnosed with ET, which was comparable to the rates in previous reports. Compared to patients with ET, patients with early/prefibrotic PMF had higher LDH values and higher frequencies of splenomegaly. Overall, myelofibrosis-free and acute myeloid leukemia-free survivals were comparable between the 2 groups. Accurate diagnosis is required to clarify the clinical features of Japanese ET patients.