Prediction of the total respiratory tract deposition fraction of an inhaled aerosol is useful in assessing its potential inhalation health risk, or its therapeutic benefit. In this communication, a simple algebraic formulation is presented through which the total deposition fraction can be predicted from dynamical, dimensionless parameters governing inertial and gravitational deposition of inhaled micrometer-scale particles. An empirical fit is first made to the total deposition fraction data of Heyder et al. [1986. Deposition of particles in the human respiratory tract in the size range 0.005-15 mu m. Journal of Aerosol Science, 17, 811-825] and Kim and Hu [2006. Total respiratory tract deposition of fine micrometer-sized particles in healthy adults: Empirical equations for gender and breathing pattern. Journal of Applied Physiology, 101, 401-412] for healthy adults, during controlled oral breathing of monodisperse micrometer-scale particles. The generality of the resulting formulation based on such dimensionless parameters is then examined by its ability to predict total deposition data measured in children [Schiller-Scotland, C. H. F., Hlawa, R., Gebhart, J., Wonne, R., & Heyder, J. (1992). Total deposition of aerosol particles in the respiratory tract of children during spontaneous and controlled mouth breathing. Journal of Aerosol Science, 23 (Suppl. 1), S457-S460], in microgravity and hypergravity [Darquenne, C., Paiva, M., West, J. B., & Prisk, G. K. (1997). Effect of microgravity and hypergravity on deposition of 0.5- to 3-mu m-diameter aerosol in the human lung. Journal of Applied Physiology, 83, 2029-2036], and in heliox as an alternative carrier gas [Darquenne, C., & Prisk, G. K. (2004). Aerosol deposition in the human respiratory tract breathing air and 80:20 heliox. Journal of Aerosol Medicine, 17, 278-285]. A single dimensionless correlation is found to provide reasonable prediction of total deposition in all these diverse cases. (c) 2006 Elsevier Ltd. All rights reserved.