A study was conducted to compare three automated systems and the Gram stain for their ability to detect significant bacteriuria. A total of 1,000 urine specimens were evaluated by Autobac MTS (General Diagnostics), Auto Microbic system (AMS; Vitek Systems, Inc.), and MS-2 (Abbott Laboratories) and compared with a semiquantitative culture plate method. Two hundred thirty-nine (23.9%) specimens had colony counts of >10(5) colony-forming units (CFU)/ml by the culture plate method (group I). Of these, 204 (85.3%) were positive by Autobac, 198 (82.8%) were positive by AMS, and 179 (74.9%) were positive by MS-2. When pure cultures of diphtheroids, lactobacilli, and viridans streptococci not group D were considered contaminants and therefore excluded, there were 118 specimens containing pure cultures of probable pathogens. The percentage of significant isolates detected was 97.4% (115 of 118) by the Gram stain, 96.6% (114 of 118) by Autobac, and 95.8% (113 of 118) by AMS and MS-2. The average detection time for all organisms was 2.2 h by Autobac, 6.1 h by AMS, and 1.8 h by MS-2; therefore, all three methods were more rapid than the 18- to 24-h standard plate culture method. One hundred sixty-one (16.1%) specimens had colony counts of 10(4) to 10(5) CFU/ml (group II). The probable pathogens not detected in this group were two (1.2%) by Autobac and MS-2 and three by AMS (1.9%). The average detection time for group II was 4.2 h by Autobac, 8.9 h by AMS, and 3.8 h by MS-2. Six hundred specimens had colony counts of <10(4) CFU/ml. Of these, 188 had colony counts equal to 10(3) and <10(4) CFU/ml (group III), and 412 cultures were below detectable limits by the standard plate method (group IV). Less than 37 and 15% of groups III and IV, respectively, were detected by instrumentation. Average detection times for groups III and IV were 4.6 and 4.8 h by Autobac, 10 and 11 h by AMS, and 4.2 and 4.4 h by MS-2. The cost of supplies and technical time with Gram stain, Autobac, and MS-2, when used as screening methods, were comparable and considerably less expensive than for the reference method. The AMS was the least expensive system when the cost for identifying probable pathogens was included.