Obesity is associated with physiological changes that can alter the pharmacokinetic parameters of many drugs. Vancomycin and the aminoglycosides are the only antibacterials that have been extensively investigated in the obese population. The apparent volume of distribution (Vd) and total body clearance of vancomycin are increased in obese patients and have a better correlation with total bodyweight (TBW) than with ideal bodyweight (IBW). The Vd of aminoglycosides is increased in obesity and can be estimated from an adjusted bodyweight that accounts for a fraction of the excess bodyweight (TBW - IBW). These observed changes in pharmacokinetic parameters of vancomycin and aminoglycosides in obese patients may necessitate a deviation from the commonly recommended dosages administered to non-obese individuals. There are limited data regarding the pharmacokinetics of other antibacterial classes in obese patients. The available information for cephalosporins suggests that dosages may need to be increased in obese patients in order to obtain similar serum and tissue concentrations as in non-obese patients. Additional pharmacokinetic studies of other antibacterial classes are required in this special patient population.