Interpretive disk diffusion breakpoints for caspofungin are proposed by evaluating 762 isolates of Candida spp., representing 10 different species obtained as part of the caspofungin clinical trials. Standardized broth microdilution reference tests were compared to the zone diameters observed with 5-microg caspofungin disks produced by two different disk manufacturers. Disk diffusion breakpoints of >or=11 mm for susceptible are proposed. Compared to results from MIC testing, these zone diameters produced error rates that were <or=0.3% for all categories. In addition, an eight-laboratory disk diffusion quality control (QC) study was performed, and QC ranges are proposed for the four QC strains recommended by the CLSI.