We investigated 107 cases of presumed microbial keratitis amongst patients presenting to the Sydney Eye Hospital between October 1986 and August 1988 to determine the frequency of infection, the common causative organisms and those factors that predispose to corneal infection. We found that 95% of culture-proven cases were caused by bacteria and that Staphylococcus epidermidis and Corynebacterium sp. accounted for 42.5% of these cases. All bacteria (except Staphylococcus epidermidis and Mycobacterium chelonae) were sensitive to gentamicin. In 62.6% of cases, cultures for bacteria and fungi were negative. Retrospective review of the clinical records of these patients showed that herpes simplex virus, contact lens wear, staphylococcal marginal keratitis and recurrent erosion were important causes of keratitis in this group.