Abstract: Objective: To identify the molecular characteristics of immunoglobulin heavy chain variable region (IGHV) genes in patients with hairy cell leukemia (HCL) and hairy cell leukemia-variant (HCL-v) and explore their correlation with clinical presentation and prognosis. Methods: A retrospective study was conducted to analyze the clinical and survival data of patients with HCL and HCL-v who completed IGHV testing in Institute of Hematology & Blood Diseases Hospital between December 2004 and January 2020. Results: In this study, 44 patients (29 with HCL and 15 with HCL-v) were included. A total of 23 rearrangements were sequenced, and VH4 and VH4-34 were the most frequently used gene family and fragment, respectively. The proportions of patients with an IGHV mutated status for HCL and HCL-v were 72.4% and 66.7%, respectively. Patients with VH4-34 rearrangement (VH4-34+) had a lower IGHV mutation rate (P<0.001) than those with other rearrangements. Patients with HCL with VH1 rearrangement had a lower incidence of splenomegaly (P=0.041), while patients with VH4-34+ or unmutated IGHV (IGHV-UM) had higher lactate dehydrogenase (VH4-34+ P=0.049, IGHV-UM P=0.022) and β₂ microglobulin (VH4-34+ P=0.039, IGHV-UM P=0.035) levels. It was noted that the BRAF V600E mutation was uncommon in patients with HCL with VH4-34+ (20% vs. 85%, P=0.012). Further univariate analysis showed that VH4-34+ was a negative prognostic factor for progression-free survival (PFS)(P=0.001) and overall survival (P=0.004) in HCL, and IGHV-UM was an adverse prognostic factor for PFS (P=0.038) in HCL-v. Conclusions: HCL and HCL-v showed preferential uses of the VH4 gene family and VH4-34 fragment. Moreover, there was obvious co-occurrence of VH4-34+ and IGHV-UM. VH4-34+ was associated with a high tumor load and poor survival in HCL, while IGHV-UM was a risk factor against PFS in HCL-v.