PURPOSE:To identify fungal keratitis patients who are at risk of a poor outcome and may benefit from closer follow-up or more aggressive treatment. DESIGN:Secondary analysis of randomized clinical trial data. METHODS:We compared the clinical outcomes of patients who had positive 6-day fungal cultures with those who did not, using backward stepwise regression with covariates for all baseline clinical characteristics. SUBJECTS:Patients presenting with a smear-positive filamentous fungal ulcer and visual acuity of 20/400 or worse, and who subsequently had a 6-day fungal culture performed at the Aravind Eye Care system (India), Lumbini Eye Hospital (Nepal), or Bharatpur Eye Hospital (Nepal). MAIN OUTCOME MEASURES:The primary outcome is rate of corneal perforation and/or the need for therapeutic penetrating keratoplasty. Secondary outcomes include 3-month best spectacle-corrected visual acuity (BSCVA), 3-month infiltrate and/or scar size, and rate of re-epithelialization. RESULTS:Patients who tested positive at their 6-day culture had twice the hazard of experiencing a corneal perforation or the need for therapeutic penetrating keratoplasty (P = .002) than those who tested negative, even after controlling for baseline ulcer characteristics. These patients also had on average 0.26 logMAR lines worse BSCVA at 3 months (P = .001). Culture positivity at day 6 was not a statistically significant predictor of 3-month infiltrate/scar-size (-0.24 mm1; P = .45) or time to re-epithelialization (hazard ratio = .81; P = .31). CONCLUSIONS:Here we identify a uniquely valuable clinical tool, day 6 culture results, for the treatment of severe fungal keratitis. Risk stratification based on repeat culture positivity is an objective way to assess response to medical therapy and identify patients who are at high risk of a poor clinical outcome. This establishes a new standard of care for severe fungal keratitis management.