OBJECTIVE: To investigate the clinicopathologic features, diagnosis, differential diagnosis and the prognosis of hairy cell leukemia (HCL). METHODS: Fifteen splenectomy specimens of HCL patients were investigated retrospectively using HE and immunohistochemistry in correlation with the follow-up information. RESULTS: (1) The male to female ratio was 2.75:1, age ranged from 36 to 68 years with a median of 47 years. The most consistent clinical feature at presentation was marked splenomegaly (100%). Other symptoms included anemia (80.0%), thrombocytopenia (60.0%), leucocytosis (53.3%), pancytopenia (20.0%) and the absence of B-symptom. (2) The proportion of hairy cells was (14.6 +/- 7.2)% in periphery blood and (47.3 +/- 23.8)% in bone marrow. The positive rate of TRAP assay was 62.5% in bone marrow; 85.7% for TPA test and the detection rate for RLC was 25% by transmission electric microscopy. The frequency of bone marrow involvement was 100%. (3) The average weight of 15 spleens was (3012 +/- 1974) g. The size of 6 spleens ranged from 16 cm x 10 cm x 5 cm to 32 cm x 20 cm x 14 cm. The white pulp of spleen showed a characteristic atrophy feature or even absent due to leukemic infiltration, predominantly involving the red pulp with some sinusoidal pattern. "Blood pool" change was an infrequent feature (3/15 cases). The nuclei of leukemic cells were round (13 cases) or bean-shaped (2 cases), nucleoli inconspicuous or disappeared. The abundant cytoplasm and prominent cell border resulted in a "fried egg" appearance. By immunohistochemistry, leukemic cells were positive for CD45RA, CD20, PAX-5, CD25, CD11c, Annexin A1 and cyclinD1, but negative for CD3 and CD43. (4) 13 cases (86.7%) have been followed-up and all are alive. Among them, 9 cases are living well more than 5 years and 7 more than 10 years. CONCLUSIONS: Splenomegaly is frequently the first manifestation of patients with HCL and occurred predominantly in the middle to elderly adults. Definite diagnosis of HCL requires a combined histological and immunohistochemical assessment of the splenectomy specimen, bone marrow biopsy and aspirate.