In a multipractice study, 577 general practitioners registered all patients to whom they prescribed antibacterial drugs. In all, 7849 patients were registered. Of these, 40% were treated for upper respiratory tract infection or otitis media. Of all patients treated for otitis media, 77% were < 15 yr of age. A total of 67% treated for sinusitis and 55% of all patients treated for tonsillitis acuta were between 15-45 yr. Significantly more women than men were treated for sinusitis, while the same number of men and women were treated for otitis media and tonsillitis acuta. Of all acid-stable and broad-spectrum penicillins prescribed in general practice, 63 and 47%, respectively, were used in the treatment of upper respiratory tract infection or otitis media. Acid-stable penicillins were prescribed for the majority of patients with otitis media and acute tonsillitis treated with antibacterial agents. Only 22% of patients with sinusitis were treated with phenoxymethylpenicillin, while 36% of these patients were treated with azidocillin. Of all azidocillin prescribed in general practice, 73% was used in the treatment of patients with sinusitis.