Hypoalbuminemia frequently occurs in Hemorrhagic Fever with Renal Syndrome (HFRS), but clinical significance of hypoalbuminemia is not well known. This study was designed to evaluate hypoalbuminemia as a marker of severity of disease in patients with HFRS. We evaluated the relationship between the level of serum albumin and clinical parameters representing the severity of disease in 144 patients with HFRS. The patients were divided into three groups based on the level of serum albumin; Group I (normal serum albumin), Group II (serum albumin <3.5 g/dL and ≥3.0 g/dL), and Group III (serum albumin <3.0 g/dL). Of the total of 144 patients, 42 patients (29.2%) were categorized as Group I, 39 patients (27.1%) as Group II, and 63 patients (43.8%) as Group III. Group III had a higher rate of incidence in episode of hypotension, pulmonary edema than did Group I and Group II. The lowest level of serum albumin was positively correlated with platelet count (r=0.505, p<0.001) and was negatively correlated with leukocyte count (r=-0.329, p<0.001), BUN (r=-0.484, p<0.001), serum creatinine (r=-0.394, p<0.001), and AST (r=-0.251, p=0.002). Our data suggest that hypoalbuminemia frequently occurs in the acute stage of HFRS, and level of serum albumin is associated with the disease severity of HFRS.