We compared readings of Kirby-Bauer plates by the Sirscan, an automated image analyzer that measures zone diameters, to those of experienced clinical microbiologists measuring zones with a hand-held caliper interfaced to a computer and with a ruler. To read plates of Escherichia coli, Morganella morganii, and Pseudomonas aeruginosa containing 12 antibiotic disks the Sirscan took 11 s; technologists took 28 s by caliper and 39 s by ruler. Reading times of four different technologists ranged from 22 to 44 s with the caliper and 10 to 12 s with Sirscan. Upon repeated testing zone size variation rarely exceeded 3 mm by caliper and 1 mm by Sirscan. Over a 4-month period, 368 clinical isolates were tested prospectively by both methods in the Clinical Microbiology Laboratory of the Miriam Hospital. There was good correlation of zone sizes for most antibiotics, but Sirscan zone diameter measurements tended to be 3 to 5 mm larger than caliper readings for ciprofloxacin, norfloxacin, aztreonam, erythromycin, clindamycin, and trimethoprim-sulfamethoxazole. Very major errors (resistant by caliper and susceptible by Sirscan) occurred with 10 of 3,770 readings (0.3%), mainly where breakpoint criteria lacked an intermediate zone. They occurred in testing staphylococci with amoxicillin-clavulanate (5 of 127 isolates, 3.9%), pseudomonas with piperacillin (1 of 28, 3.6%), coagulase-negative staphylococci with oxacillin (2 of 74, 2.7%), gram-negative bacilli with cefuroxime (1 of 209, 0.5%), and mixed species with trimethoprim-sulfamethoxazole (1 of 366, 0.3%). The Sirscan zone reader facilitates accurate, fully quantitative susceptibility testing in clinical microbiology laboratories.