A 57-year-old man presented with 2 months of weakness, fatigue, gingival bleeding, and cachexia. Examination revealed right mydriasis, bilateral retinal hemorrhages, diffuse lung crackles, a tense abdomen, and normal lymph nodes. Increased blood viscosity prevented laboratory draw before hydration. Blood count demonstrated 14.6 × 10 9/L leukocytes with 85% lymphocytes, 8.2 g/dL hemoglobin, and 130 × 10 9/L platelets; a blood smear showed lymphocytosis and rouleaux formation. Serum viscosity was 11.0 centipoise (normal, 1.5-1.9). A 2.6-g/dL immunoglobulin M κ (IgM-κ) paraprotein was detected. Positron emission tomography/computed tomography showed increased uptake of [ 18F]fluorodeoxyglucose in the enlarged spleen and in bone marrow. Splenic marginal zone lymphoma (SMZL) was diagnosed after bone marrow examination; the MYD88 L265P mutation was absent.