Fifty-three patients consecutively admitted to Moorfields Eye Hospital for treatment of suspected microbial keratitis were examined to identify predisposing factors. The principal associations were pre-existing corneal disease (22 patients (41.5%] and contact lens wear (22 patients (41.5%]. In 13 cases (25%) contact lens wear was the only factor in patients with otherwise healthy eyes using contact lenses as an alternative to spectacles. Gram-negative keratitis was more frequent in the lens wearers, with the exception of therapeutic lens users, than in other patients (p = 0.0006) and Pseudomonas aeruginosa caused keratitis in cosmetic soft lens users more frequently (p = 0.001). There was no correlation between lens handling or solution contamination in three extended wear soft-lens users. This implies that some soft-lens wearers may be infected by Gram-negative organisms from environmental sources other than contaminated lens care materials. Gram-negative keratitis is strongly associated with contact lens wear, and the diagnosis must be considered in any contact lens user with an acutely painful red eye.