The efficacy and tolerance of two different dosage regimens of pivampicillin was compared in 155 patients with acute sinusitis and in 56 patients with otitis media. Patients received a 10-day course of either 500 mg pivampicillin twice daily, or 350 mg pivampicillin 3-times a day. Out of the 155 patients with sinusitis, 68 patients received pivampicillin at the time of diagnosis, whilst in a further 87 patients treatment was given, on average, 7 days after the diagnosis was made. Approximately 78% of the patients had Proetz suction and 17% used nasal drops or spray. Among 77 patients given the 500 mg dose, 88% were cured, 8% improved and only 4% failed to respond. Ninety-one per cent of 70 evaluated patients receiving the 350 mg dose were cured and 9% were improved. Paracentesis was performed on 43 of the 56 patients with otitis media and spontaneous perforation occurred in 7 patients. Eight patients received supplementary treatment with ear drops. Ninety-one per cent of 32 evaluated patients on the 500 mg dose were cured and 9% improved. The corresponding response rate in 20 evaluated patients given the 350 mg tablets was 80% cured, 15% improved and 5% of patients failed to respond. There was no difference in the frequency of side-effects between the two regimens. Approximately 4% of the patients had gastro-intestinal side-effects or skin rashes and in only 6 patients did these necessitate withdrawal of therapy. The twice daily regimen is recommended because of increased patient compliance.