Rupture of the spleen occurred in two patients (a man of 32 and a girl of 16 years) with serologically proven infectious mononucleosis. No history of trauma as a cause of the rupture could be elicited in either patient, despite intensive questioning. Abdominal symptoms were left-sided upper abdominal pain radiating into the left shoulder (Kehr's sign). The suspected diagnosis was confirmed by ultrasound demonstration of free abdominal fluid and abnormal splenic structure. Splenectomy had to be performed in both patients.