A polydisperse aerosol inhalation system has been developed to measure particle deposition in the lungs of human subjects. Nebulizers are used to generate aerosols with mass median aerodynamic diameters from 0.3 to 3 mum, and geometric standard deviations of 1.8-2.0. Inspired aerosol is drawn from a holding bag, passes through a sliding valve and a pneumotachograph, and enters a heated mouthpiece. Exhaled aerosol passes from the mouthpiece and pneumotachograph, through a second sliding valve and is collected in a heated sample bag. Inhalation and exhalation valves trigger automatically with change in flow direction through the pneumotachograph. Complete size distributions of inhaled and exhaled aerosol are measured by an aerodynamic particle sizer and a scanning mobility particle sizer. Fractional particle deposition of a test aerosol in the lung is determined by comparing inhaled and exhaled aerosol size fractions. Total deposition is determined from the sum of the fractional depositions. This new system precludes the need for monodisperse aerosol series to simulate polydisperse aerosol data, thus substantially reducing both study length and subject exposure. (C) 2002 Elsevier Science Ltd. All rights reserved.