The present study aims at an assessment of hepato-splenomegaly in infectious mononucleosis (IM). In 29 patients admitted to the ENT department with IM, based on the typical clinical and laboratory findings, including a positive mononucleosis test in most cases, the size of the liver and spleen was estimated by ultrasonic scanning on days 1, 3, 5, 10, 20, 30, 90 and 120 after admission. A control group of eight patients with peritonsillar abscess was included for comparison. The results showed that all patients had an enlarged spleen (mean enlargement: 50–60 per cent) but only a few were palpable. Half of the patients had enlargement of the liver (5–20 per cent), which was palpable in only 8 per cent. There was no correlation between the size of the spleen and that of the liver, not between the changes in the size of these organs. There was no enlargement of the liver or spleen in the control group. No correlation was found between the size, or changes in the size, of the organs and blood values such as lactatdehydrogenase and aspartatamino-transferase. There is, however, a striking parallelism between the curves for these parameters, which might indicate that the organs as well as the blood tests return to normal within 28 days. If this holds true, our warning to abstain from physical exercise and alcoholic intake may be limited to a period of about 1 month.