Splenic rupture is a rare but potentially fatal complication of infectious mononucleosis. Athletes returning to contact sports following infectious mononucleosis are at potential risk of splenic rupture secondary to abdominal trauma. No clear consensus exists as to when it is safe to allow these athletes to return to contact sports. Suggested periods of abstinence have ranged from 2 weeks to 6 months. We outline our experiences with the use of abdominal ultrasonography at 1 month after the diagnosis of infectious mononucleosis as a means of determining when athletes can safely return to contact sports. Our study group was made up of 19 such patients (mean age: 16.7 yr). We found that 16 of these patients (84%) had normal splenic dimensions on ultrasonography 1 month after diagnosis, and they were therefore allowed to return to contact sports. While the remaining 3 patients had an enlarged spleen at 1 month, their splenic dimensions had all returned to normal when ultrasonographic examination was repeated at 2 months postdiagnosis. We conclude that serial abdominal ultrasonography allows for informed decision making in determining when athletes can safely return to contact sports following infectious mononucleosis.