The incidence of extended-spectrum beta-lactamases (ESBLs) was analyzed in Enterobacteriaceae population circulating in the Upper Silesian Child and Mother Health Center in Katowice (USC&MHC). Altogether 1164 clinical specimens, collected from children hospitalized in 8 different hospital units of USC&MHC were investigated. Five hundred and eighty-five clinical isolates of the family Enterobacteriaceae were identified in specimens collected from 403 patients. Two hundred and twenty-nine Enterobacteriaceae strains (39%) isolated from 162 patients were found to be putative ESBL producers as revealed by double-disc synergy (DDS) test. ESBL activity was the most prevalent in the population of Klebsiella pneumoniae (77%), followed by Klebsiella oxytoca (50%), Serratia marcescens (43%), Escherichia coli (30%), Enterobacter spp. (18%) and Proteus mirabilis (12%). ESBL producers demonstrated also wide resistance to the non-beta-lactam antimicrobial co-trimoxazole (93%) and the aminoglycosides netilmicin (88%), gentamicin (84%) and amikacin (79%).