Objective: To describe quantitatively the variability associated to the pharmacokinetic (PK) processes of clarithromycin (CLA) in Mexican hospitalized patients with respiratory infection and to determine whether the 6-beta-hydroxycortisol (6 beta-OHC)/cortisol ratio, among other factors would partially explain such variability. Materials and Methods: Fifty three patients aged >18 years with respiratory disease treated with CLA were included in the study. An average of 3 blood samples per patient were obtained at approximately the following Times After Dosing (TAD): 0.5, 1.25, 2, 3, 4, 6, 9 and 12 h. Clarithromycin was given orally or i.v., twice daily at the dose of 500 mg. Around the same times at which blood samples were collected, one urine sample was obtained for determining the 6 beta-OHC/cortisol ratio. The serum concentration vs time data of CLA were modeled using the population approach with NONMEM 7.2. Results: A one-compartment disposition model with first-order rate of absorption and concentration independent distribution and elimination provided a reasonable description of the data. Absolute bioavailability of CLA was not different from 1 (p>0.05). The population estimate of total clearance was 14.6 L h(-1), lower than that reported previously for healthy volunteers. Final population model included body weight as the unique covariate affecting the apparent volume of distribution. Conclusion: The study population showed a total clearance lower than that reported for healthy volunteers from other countries, probably due to the low activity of CYP3A determined in this population. However, the CYP3A activity level did not result as a significative covariable of the CLA total clearance.