During recent years, a significantly increased incidence of acute mastoiditis was observed at the University Children's Hospital, Tübingen, Germany (1975-1979: 1.4 patients/year; 1987-1992: 4.2 patients/year; p < 0.05). We therefore reviewed the records of all patients with acute mastoiditis (n = 58) that had been treated at the Children's Hospital between 1975 and 1992 and at the Clinic of Otorhinolaryngology between 1978 and 1992. The male to female ratio was 1.8:1 and 60.4% of the patients were younger than 24 months. Retroauricular swelling was more often observed (n = 49) than protrusion of the pinna (n = 45) and retroauricular redness (n = 38). Streptococcus pneumoniae was the most frequently isolated pathogen. Several factors predisposing for the development of acute mastoiditis were identified. These included withholding antimicrobials for treatment of the preceding episode of otitis media; use of suboptimal agents for therapy of otitis media (penicillin and, possibly, erythromycin ethylsuccinate); and insufficient duration of treatment.