Class of 2006 Abstract / Objectives: To develop a new equation to predict creatinine clearance specific for cystic fibrosis patients. Methods: A literature review was performed to capture data on the daily creatinine excretion in CF patients in relation to age, weight, height, and other physiologic variables. Nonlinear mixed effect modeling was then used to develop an equation to estimate creatinine clearance using individual covariates. The performance of the new equation developed was compared to the Cockcroft and Gault method in a CF population (age > 16 years). Results: A database of individual patient data from a previously published study of 19 patients was created. Significant covariates for model development included actual body weight, sex, and serum creatinine. The final candidate model was: 5.62× ABW0.67 CrCl = sCr(mg / dl) × 0.649( female) Conclusions: The results of the mean absolute error and root mean squared error calculations show that the new equation resulted in less bias and better precision than Cockcroft-Gault, Jeliffe I, and Jeliffe II based on the limited data available. However, these conclusions are limited in that the only evaluation data available was the same data that was used for model development.