Combination antimicrobial regimens consisting of an agent with activity against gram-negative bacilli (an aminoglycoside) plus an agent with anaerobic activity (usually clindamycin or metronidazole) have traditionally been accepted as the standards for the treatment of intra-abdominal infection. Because of the problems of nephrotoxicity and ototoxicity in patients treated with aminoglycosides, clinical trials have been conducted using alternative combination therapy (e.g., aztreonam plus clindamycin) or single beta-lactam antimicrobial agents. Most clinical trials of intra-abdominal infections have been conducted in relatively small patient populations with a variety of low- and high-risk patients. The newer regimens have demonstrated efficacy equivalent to traditional combination therapy in selected patient populations. When selecting an antimicrobial regimen for treatment of intra-abdominal infection, multiple issues should be considered, including demonstrated efficacy in clinical trials, potential for adverse effects, and cost.