Risk factors in contact lens related suppurative keratitis were investigated. Over 12 months, sixty contact lens wearers presented with presumed microbial keratitis and 147 presented with presumed sterile keratitis. The control group consisted of contact lens wearers without lens related disorders (n = 507). The relative risk of microbial keratitis for extended wear soft contact lens users were 36.8 times (12.6-107.6) that of rigid gas permeable (RGP) contact lens users; while the relative risk of sterile keratitis among extended wear soft lens users was 4.6 times (2.2-9.9) that of RGP lens users. For daily wear soft contact lens users, the relative risk of microbial keratitis was 4.2 times (1.1-16.0) that of RGP lens wearers; for sterile keratitis the relative risk was 2.3 times (1.3-4.3) greater than RGP contact lens users. A multivariable logistic regression analysis was used to examine associated risk factors for daily and extended wear soft lens users. The type of disinfection system and its frequency of use were associated with both microbial keratitis and sterile keratitis in daily wear users. Duration of overnight wear and gender were associated with microbial keratitis, while a patient's age and socioeconomic class were associated with sterile keratitis. Different relative risks and risk factors for microbial keratitis and sterile keratitis support the use of a clinical definition to differentiate the diseases.