The terms periorbital (preseptal) and orbital cellulitis are often used interchangeably, obscuring important differences in their pathogenesis, bacterial etiology, clinical presentation and appropriate therapy. A review of 56 cases of periorbital cellulitis indicated that the patients could be divided into three groups: Group 1, cases secondary to paranasal sinusitis, more correctly termed inflammatory edema because the periorbital swelling is due to venous obstruction (a specific bacterial etiology is rarely documented in these patients because aspiration and culture of the sinuses are usually not performed and the infection is not associated with bacterial invasion of either the soft tissue or the blood stream); Group 2, cases associated with disruption of local skin integrity which are usually due to Staphylococcus aureus or Group A streptococci; and Group 3, cases associated with bacteremia usually occurring in infants and young children without other apparent foci of infection and caused by Haemophilus influenzae type b or Streptococcus pneumoniae. Actual infection of the orbital contents (orbital cellulitis or abscess), marked by proptosis and ophthalmoplegia, is rare and is due either to advanced purulent sinusitis or to penetrating orbital trauma.