BACKGROUND Chronic neutrophilic leukemia (CNL) is a rare bone marrow proliferative tumor and a heterogeneous disorder. In 2016, the World Health Organization included activating mutations in the CSF3R gene as one of the diagnostic criteria, with CSF3R T618I being the most common mutation. The disease is often accompanied by splenomegaly, but no developmental abnormalities and significant reticular fibrosis, and no Ph chromosome and BCR-ABL fusion gene. So, it is difficult to diagnose at the first presentation in the absence of classical symptoms. Herein we describe a rare CNL patient without splenomegaly whose initial diagnostic clue was neutrophilic hyperactivity. CASE SUMMARY The patient is an 80-year-old Han Chinese man who presented with one month of fatigue and fatigue aggravation in the last half of the month. He had no splenomegaly, but had persistent hypofibrinogenemia, obvious skin bleeding, and hemoptysis, and required repeated infusion of fibrinogen therapy. After many relevant laboratory examinations, histopathological examination, and sequencing analysis, the patient was finally diagnosed with CNL [ CSF3R T618I positive: c.1853C>T (p.T618I) and c.2514T>A (p.C838)]. CONCLUSION The physical examination and blood test for tumor-related genes are insufficient to establish a diagnosis of CNL. Splenomegaly is not that important, but hyperplasia of interstitial neutrophil system and activating mutations in CSF3R are important clues to CNL diagnosis.