Liver involvement during infectious mononucleosis is common, but jaundice is considered rare. This study aimed to investigate serum bilirubin concentrations in patients with infectious mononucleosis and immune abnormalities associated with jaundice. We report on an adult patient with monoclonal B lymphocytosis and IgM-lambda gammopathy who developed a severe icteric hepatitis during infectious mononucleosis. We then reviewed the clinical records of 389 patients admitted to the hospital with infectious mononucleosis between 1995 and 2018 (51.7% male patients; median age, 19 years; range, 15-87 years) with focus on liver abnormalities and associated factors. Fifty-nine patients (15.1%) had serum bilirubin concentrations between 1.5 and 3 mg/dL, and 47 patients (12.0%) had serum bilirubin >3 mg/dL. Patients with increased bilirubin concentrations had a distinct clinical presentation, with more frequent abdominal pain, nausea and vomiting, and less frequent sore throat than patients with normal bilirubin. Age and sex were not significantly different for the patients with increased and normal serum bilirubin concentrations. The patients with increased serum bilirubin concentrations showed higher levels of immune activation markers than the patients with normal bilirubin, including blood lymphocyte counts, serum IgM, and β2-microglobulin concentrations. Heterophile antibody-positive patients (88.6%) showed similar bilirubin concentrations but higher aspartate aminotransferase and alkaline phosphatase levels than their heterophile-negative counterparts. Serum bilirubin elevations normalized quickly during follow-up. Transient hyperbilirubinemia is common during severe (in-hospital) infectious mononucleosis in adult patients. Patients with hyperbilirubinemia have less frequent pharyngitis symptoms and more frequent abdominal symptoms. Hyperbilirubinemia during infectious mononucleosis is associated with immune activation markers.