The objective of this study was to evaluate relative efficiency in vitro of four reusable breath-enhanced nebulizers (Pari LC Star, Medic-Aid Ventstream, Devilbiss PermaNeb, Salter Ultramist), and to integrate the in vitro performance data of the nebulizers with the respiratory patterns of four cystic fibrosis (CF) patients to compare efficiency in vivo of each device for each individual patient. Six nebulizers of each type were used to nebulize a solution of 2.5 mg (0.5 mL) albuterol with 3.5 mL of 0.9% saline. Total albuterol output and the rate of albuterol output of each device were measured until end-nebulization and for 4 min, respectively, using entrained flows from 0 to 20 L/min through the inspiratory valve of the device. Particle size distributions and the respirable fraction (RF) were evaluated by laser diffraction technique. Regression analysis of the change in rate of output and change in RF values with inspiratory flows was done to characterize each nebulizer's performance over the complete range of interest. Actual breath tracings of four CF patients were integrated with the equations specific to the in vitro performance of each nebulizer and in vivo nebulizer efficiency was calculated. The change in efficiency in vitro from 0 to 20 L/min flow, respectively, was highest for the Star (44-57%) and lowest for the Ultramist (13-15%). The mean predicted efficiency in vivo for the Star was threefold that of the Ultramist. Although all four nebulizers are breath-enhanced in design, clearly there are measurable differences in the performance and efficiency of each type. The Pari LC Star nebulizer has proven to be the nebulizer of choice among the devices tested.