Bacteria growing as biofilms are less susceptible to antimicrobial agents than free-living cells. Several infectious processes are now recognised to be related to biofilm formation. Pseudomonas aeruginosa is a known biofilm-producing organism and often resistant to several antibiotics. To assess the relationship between biofilm-forming capabilities and antibiotic resistance phenotypes in P. aeruginosa clinical isolates, we evaluated the ability of different phenotypes to form biofilms using 162 clinical isolates of P. aeruginosa. Only 14% of strains produced biofilm after an 8-h incubation and 8% after 24h; the latter group was slightly more multi-resistant, particularly to piperacillin-tazobactam and imipenem. The frequency of 8-h biofilm-formers was higher among isolates resistant to four agents, and that of 24-h biofilm-formers higher among isolates resistant to six antibiotics. Plasmid-content was not related to biofilm formation. In addition to the protection against antibiotics that bacteria gain by growing in a biofilm, clinical isolates forming biofilms seem to accumulate more resistance phenotypes. Bacteria in biofilms may survive antibiotic exposure not only because of the protective effect of biofilm itself, but also because of the increased frequency of resistance traits present.