The effect of peroral penicillin-V (55 mg/kg/day in 7 days) on acute otitis media was studied in 149 children between the ages of 1 and 10 years in a double-blind, placebo-controlled investigation. The parameters of the disease employed were symptom scores for earache, fever and common cold, the use of analgetics, otoscopy, as well as tympanometry. The children were followed-up for 3 months. Penicillin had no effect on fever and common cold, but earache was significantly reduced on the 2nd day of treatment. The acute course of the disease was satisfactory in 69% of the children in the placebo group and in 86% in the penicillin group. In patients with pneumococci or haemolytic streptococci in the nasopharynx, the pain disappeared after 1-2 doses of penicillin, whereas the treatment had no effect in children with Haemophilus influenzae. There was no difference between the penicillin and the placebo groups with regard to the results of otoscopy and tympanometry after 1 week, 1 month and 3 months. No serious complications were observed. It is concluded that an attitude of "masterly inactivity" with regard to the treatment of acute otitis media is justifiable, provided sufficient analgesic treatment is given and also that the patient can be closely followed. As there are still many unanswered questions more controlled investigations are warranted.