Watchful waiting was recommended as an option for uncomplicated episodes of acute otitis media in Sweden in the year 2000. Concern was raised that these recommendations would lead to a higher incidence of acute mastoiditis. The aim of this study was to map the occurrence, treatment policy and the clinical course of mastoiditis before and after the new treatment recommendations were introduced. Included in the study were all patients (adults and children) who were admitted to two ENT departments in southern Sweden for acute mastoiditis from 1996 to 2005. A total of 42 cases of mastoiditis were identified: 23 during the first period of 1996-2000 and 19 during 2001-2005. Mastoidectomy was performed in 14 patients during the first period and in 8 during the second period. As much as 39% of mastoiditis patients received antibiotics before hospital care, but had no improvement. There was no indication that the number of patients with acute mastoiditis was increasing after new treatment recommendation of AOM. There was no increase in the occurrence of mastoidectomy. Severe complications of mastoiditis were rare. Although there were potentially threatening complications of mastoiditis in the study, these did not lead to sequelae. It is important to follow up the consequences when treatment recommendations of AOM are changed.