A total of 103 isolates of Pseudomonas aeruginosa were studied to compare the in vitro effectiveness of three beta-lactam antibiotics (piperacillin, ticarcillin, and mezlocillin) when used alone and in combination with four aminoglycosides (tobramycin, gentamicin, amikacin, and netilmicin). All drugs were tested as single agents against a standard inoculum (5 x 105 CFU/ml). The three antipseudomonal penicillins were also tested against the isolates at a higher inoculum concentration (107 CFU/ml). Synergy testing was performed by the two-dimensional checkerboard method and was defined by (i) a fractional bactericidal index of ≤0.5 and (ii) bacterial killing accomplished at antibiotic concentrations no greater than those achievable serum. All combinations were assessed for synergy. The degree of synergy was further analyzed by dividing the isolates into groups based on their susceptibility and resistance to the individual agents in the combination. The overall effectiveness of the various aminoglycoside-antipseudomonal penicillin combinations was assessed regarding their ability to kill the isolates either as single agents or through synergy. Piperacillin was the most active antipseudomonal penicillin, and tobramycin and amikacin were the most active aminoglycosides when used as single agents. When tested against isolates at a higher inoculum concentration, ticarcillin was significantly more active than the other beta-lactams. The highest degree of overall synergy was noted with gentamicin-ticarcillin (78.2% of strains) and amikacin-piperacillin (77% of stains). When assessed for overall effectiveness, all combinations containing amikacin were the most active. The combination of amikacin-piperacillin was the most effective, with activity against 96% of all isolates.