Sixty four Stenotrophomonas maltophilia and thirty eight other non-fermentative gram negative bacilli were studied in order to determine the sensitivity and specificity of aztreonam-amoxicillin clavulanic acid synergy for the diagnosis of S.maltophilia. We found that at a distance of 15 mm this synergy test was 100% spesific and 94% sensitive. In our opinion, this test can be used as a routine test for the diagnosis of S.maltophilia.