We evaluated the ability of 60 Italian clinical microbiology laboratories in detecting and reporting beta-lactam resistance phenotypes in Enterobacteriaceae. Laboratories received 5 well-characterized isolates producing extended-spectrum beta-lactamases (ESBLs), 2 hyperproducers of chromosomal enzymes, and 3 quality control strains. The performances in antimicrobial susceptibility testing (AST) were different depending on the species and type of ESBL produced. High rates of very major errors (up to 56%) were observed for ESBL producers when testing cephalosporins and aztreonam, especially in the case of CTX-M-1-producing Escherichia coli and TEM-52-producing Proteus mirabilis. Isolates hyperproducing chromosomal enzymes were erroneously reported as ESBL producers in approximately 20% of cases. Detection of ESBLs is still a problem for clinical microbiology laboratories. Overall, performances in AST appear to be better with Klebsiella spp. producing well-known enzymes (e.g., SHV type) than with strains producing emerging enzymes (e.g., CTX-M type) or organisms not well recognized as ESBL producers (e.g., P. mirabilis).