The clinical features of infection in patients from whomStreptococcus milleri was isolated were analysed in an attempt to determine the clinical significance of this organism. During a four-year periodStreptococcus milleri was isolated from 232 hospitalized patients. In 44 patientsStreptococcus milleri was isolated in pure culture, in 45 patients together with obligate anaerobes, and in 143 patients together with aerobes with or without anaerobes. The 82 patients in whom isolation ofStreptococcus milleri was considered significant had the following infections: bacteremia (8 patients), brain abscess (2), pleural empyema (9), lung abscess (1), maxillary sinusitis (7), intra-abdominal abscess (53), infection of pacemaker (1) and infection of vascular graft (1). The 150 patients in whom isolation ofStreptococcus milleri was considered of questionable significance had the following: upper respiratory tract infection (12 patients), lower respiratory tract infection (6), acute cholecystitis (8), soft tissue abscess, cellulitis and surgical wound infections (83), bone and joint infections (14), genital infection (25) and miscellaneous infections (2). The overall incidence of infection withStrepococcus milleri was five cases per 1000 admissions. The study showed thatStreptococcus milleri is of clinical significance not only in suppurative infections, as previously reported, but also in acute maxillary sinusitis and infection of implant material.