Background: We reviewed patients with fever of unknown origin (FUO) and splenomegaly and assessed the diagnostic value of splenectomy and measured risk factors suggestive of an underlying lymphoma. Methods: FUO patients (n = 83) who had splenomegaly and underwent splenectomy were enrolled into this retrospective single-center study. Clinical presentations were documented and risk factors suggestive of an underlying lymphoma were tested. Results: Seventy-four patients (89.2%) had a diagnosis of lymphoma or not after splenectomy and follow-up. Of those (55.4%) diagnosed with lymphoma, 29 had B-cell non-Hodgkin lymphoma and 12 had T-cell non-Hodgkin lymphoma. The remaining 33 (44.6%) had diseases other than lymphoma. Using multivariate logistic analysis, the following 3 independent risk factors were found to be related to a final diagnosis of lymphoma: age (continuous) (HR 1.086; 95% CI 1.033-1.141; p = 0.001), massively enlarged spleen (HR 7.797; 95% CI 1.267-47.959; p = 0.027), and enlarged intra-abdominal lymph nodes (HR 63.925; 95% CI 7.962-513.219; p < 0.001). The calibration of the model was satisfactory (p = 0.248 using the Hosmer-Lemeshow test), and the discrimination power was good (area under the receiver operating characteristic curve 0.925; 95% CI 0.863-0.987). Conclusions: Splenectomy is an effective diagnostic procedure for patients with FUO and splenomegaly and lymphoma is a common cause. Older age, a massively enlarged spleen, and enlarged intra-abdominal lymph nodes are risk factors suggesting an underlying lymphoma, and surgery for high-risk patients should be considered.