Objective: To quantitatively assess the degree and rate of splenic enlargement and the time required for regression of splenic enlargement in collegiate athletes diagnosed with acute infectious mononucleosis (IM). Design: Prospective cohort study. Setting: Academic Medical Center(s) outpatient sports medicine clinic. Study participants: Volunteer Division I University athletes. Interventions: A limited abdominal ultrasound was performed on each participant by a licensed and experienced ultrasonographer. Splenic measurements were taken to assess maximum splenic length. Athletes who were subsequently diagnosed with infectious mononucleosis (clinical illness and a positive monospot) underwent serial splenic ultrasounds and physical exams (weekly) until resolution of clinical symptoms and splenic enlargement (as determined by ultrasound measurements). Main outcome measures: Per cent enlargement of spleen size (length) from baseline. Time (in days) from onset of clinical illness to maximum splenic length. Time (in days) required for resolution of splenic enlargement. Results: 20 subjects were diagnosed with acute IM during a 5 year time period. Maximum splenic length increased a mean of 33.6% (SD 19.9%) from baseline values. Peak splenic enlargement was reached a mean of 12.3 (SD 5.1) days from onset of clinical illness. A linear model demonstrated that spleen size decreases approximately 1% per day after reaching peak splenic enlargement. Conclusions: The majority of athletes with IM experience a moderate degree of splenomegaly. Peak splenic enlargement occurs within 2 weeks from the time of symptom onset in most cases, but may extend to 3.5 weeks. The rate of splenic enlargement appears to be predictable for an individual who develops IM. Ultrasonographic data further show that splenomegaly associated with acute IM infection resolves within 4-6 weeks of symptom onset in the majority of cases.