Seven patients with acute mastoiditis are reported. They were treated at Chiba Children's Hospital from 1991 to 1996. Patients included one 4-year-old child and 6 one-year-old infants. Except the 4-year-old girl whose body weight was 17 kg, all infants body weights were under 11 kgs. Chief complaints were ear discharge, retroauricular swelling and high fever (> 38 degrees C). Streptococcus pneumoniae was isolated in the 6 infants. One patient was diagnosed with penicillin-sensitive Streptococcus pneumoniae (PSSP), three patients were diagnosed with penicillin-insensitive Streptococcus pneumoniae (PISP) and the remaining patients were diagnosed with penicillin-resistant Streptococcus pneumoniae (PRSP). All 7 patients had developed otitis media subsequent to acute mastoiditis. The otitis media or earlier upper or lower respiratory infections had been treated with penicillin or cephalosporin by oral administration for more than a week. As to the therapy of acute mastoiditis, all of the patients received intravenous antibiotics. One (PSSP case) treated in 1991 received ABPC, and the others treated after 1995 received PAPM/BP for 5 to 8 days. None of the patients required mastoidectomy. In conclusion, antibiotics such as PAPM/BP with myringotomy were administered intravenously for the first treatment of acute mastoiditis in infants, as PISP and PRSP were commonly detected in the culture. We also considered that the proper administration of antibiotics could make the infants free from operation.