Two hundred twenty-seven cases of microbial keratitis reported in nonreferral county practice were studied. The staphylococci, Pseudomonas aeruginosa and Streptococcus pneumoniae, were the major isolates. A multivariate statistical model was developed to evaluate possible predisposing and outcome determinants. Several racial and age-related relationships were shown. The interaction of numerous local ocular and systemic factors played a fundamental role in causing disease. The authors found significant association between S. pneumoniae and topical steroid use, and direct and indirect linkage of S. aureus with diabetes and trauma, respectively. S. pneumoniae and Moraxella were risk factors for major complications (24% of cases); S. pneumoniae was related to enucleation and late perforation. Corneal exposure and prior topical steroids were associated with prolonged hospital stays. Hypopyon was associated with pneumococcal infection, 60 years of age or older, and trauma. The identification of groups at high-risk for microbial keratitis and problems of preventive management are discussed.