Purpose. The purpose of this study was to determine the factors that contribute to corneal staining in hydrogel lens wearers by examining the following variables: type of lens worn, wearing time and lens replacement schedule, lens care system, and topical and systemic medication use. Methods. Five hundred full-time, successful hydrogel contact lens wearers were evaluated for corneal fluorescein staining at 20 clinical sites. Corneal staining was evaluated in five regions and was graded on a scale of 0 (no staining) to 4 (severe staining) in 0.5 steps for severity. Results. Corneal fluorescein staining was present in at least one eye in 55.7% of the hydrogel lens wearers examined in this study, and 8.0% had moderate-to-severe staining (defined as a cumulative staining score of ≥3 with at least one quadrant score ≥2). Staining was most severe for the inferior portion of the cornea (mean ± SD; OD: 0.34 ± 0.57, OS: 0.30 ± 0.54). All other areas of the cornea showed lesser amounts of staining, all of which were approximately equal (mean range, 0.13 to 0.16). Staining was observed in two or more quadrants in 24% of right eyes and 22.4% of left eyes. When questioned about care system compliance, 81.1% of subjects reported compliance. Patients who were noncompliant with their lens care system (X2 p = 0.0037), used rewetting drops (X2 p = 0.0005), or wore conventional lenses without a planned replacement schedule (X2 p = 0.0008) were more likely to have some degree of corneal staining. Noncompliance with care system (X2 p = 0.0147), replacement schedule (X2 p = 0.0039), and lens power (X2 p = 0.0224) were associated with moderate-to-severe staining. The following factors were not significantly associated with corneal staining: patient age, gender, medication use including oral contraceptives, type of wear (extended, flexible, or daily), average wearing time, wearing time at the examination, lens type (spherical or toric), water content of lenses (high or low water content), type of care system, use of enzymes, and pinching the contact lens directly off the cornea. Conclusions. Staining occurs to some extent in many hydrogel contact lens patients, and is influenced by many factors. Moderate-to-severe staining, which may be more clinically significant, is associated with noncompliance with care system, a conventional replacement schedule, and lens power.