The dispersion of aerosol boluses in the human lungs has been studied in health and disease, usually as a means of investigating convective mixing. However, there are limited data on the roles of critical factors, such as the volume of inhaled boluses, lung inflation, and gender on dispersion. To examine these factors, we measured the difference in volume variance between exhaled and inhaled boluses (sigma 2V) of a 0.5-micron aerosol in 11 healthy male and 12 healthy female subjects as a function of tidal volume (VT = 1,000 and 1,500 ml in females and 1,000 and 2,000 ml in males), bolus penetration volume (Vi at 250-ml increments over each VT), and bolus volume (target VBol = 75, 150, and 300 ml). Analysis of variance showed marginally significant gender effects (P = 0.073) on sigma 2V, with sigma 2V greater in males than in females. There was also a significant effect of VBol on sigma 2V (P < 0.001). A Vi-dependent mean volume shift between inhaled and exhaled boluses (delta V) was observed at all Vi except 500 ml. The observation of gender and VBol effects and the existence of a nonzero delta V suggest that convective mixing mechanisms other than longitudinal dispersion alone occur in the healthy lung. The lack of VT dependence suggests a minimal role of lung inflation above functional residual capacity on dispersion. The dependence of sigma 2V on Vi2 up to 1,750 ml and minimal VBol effects demonstrates that convective mixing processes continue far into the gas exchange regions of the lung and support a significant role for axial streaming.