To establish the predisposing factors, microbial profiles, demographics of patients and clinical outcomes of microbial keratitis at a tertiary eye center in northern Thailand. Patients admitted to Chiang Mai University Hospital with suspected microbial keratitis (non-viral) were studied retrospectively over a 36-month period (April 2003-March 2006, respectively) (n = 305 cases/310 eyes). Predisposing factors, causative organisms, patients' demographic and treatment outcomes were analyzed. Ocular trauma was the predisposing factor (43.9%) identified most commonly, followed by undetermined causes (16.1%), ocular surface diseases (13.20%), multiple factors (9.40%), systemic disease (6.80%), ocular surgery (3.9%) and use of contact lens (3.5%). Cultures of corneal scraping were positive in 25.6% of cases. Both bacteria and fungi were common pathogens (49.3% and 46.3%, respectively). Pseudomonas aeruginosa (14.90%) and fusarium spp. (26.90%) was the most common bacterial and fungal pathogen, respectively. Forty-one percent of eyes underwent surgery and the most common procedure was scleral patch graft (39.8%). A statistically significant predictor of poor visual outcome was an ulcer larger than 6 mm (OR 3.08, p = 0.002). Ocular trauma was the most common predisposing factor leading to microbial keratitis. Both bacteria and fungi were common pathogens. A large lesion at presentation was a significant predictor for poor visual outcome.