The incidence of mastoiditis, a known complication of otitis media, may reflect the incidence and treatment of otitis media. The aim of the study was to evaluate the incidence of mastoiditis in Iceland, especially in children, and the possible correlation with antibiotic usage. Patients with mastoiditis during 1984-2002 were identified and information on antibiotic usage in children in the y 1989-2002 was obtained. 84 patients were diagnosed with mastoiditis during 1984-2002, 52 (62%) of whom were less than 18 y of age. 26 (50%) children were less than 3 y of age. During the y 1999-2002, 28 children were diagnosed with mastoiditis, of whom 15 (54%) were diagnosed with otitis media within a week prior to admission. 11 (73%) were treated with antibiotics and 4 (27%) were not. During 1989-2002 a correlation was detected between decreased antibiotic usage in children and increased incidence of mastoiditis (r=-0.68; p=0.007). Following changes in guidelines for antibiotic prescriptions for otitis media in Iceland during the 1990s, antibiotic usage decreased but the incidence of mastoiditis increased. It is uncertain if this is a causal relationship. It is important to treat otitis media correctly while being alert for complications, especially in young children.