The purpose of this study was to determine and understand the effect of the polydispersity of fine lactose (FL) on the dispersion of salmeterol xinafoate (SX) from SX-coarse lactose mixtures for inhalation. SX mixtures were prepared using validated laboratory mixing. The in vitro deposition of SX was measured using twin-stage impinger and SX analysed using high performance liquid chromatography. The distributions of FL included both cohesive to non-cohesive fractions. Reduction in the span of the FL distributions with a volume median diameters (VMD) about 7 i??m showed no significant difference in the fine particle fraction (FPF) of SX (P>0.05), while reduced FPF of SX was observed with the reduction in the span of FL with VMD about 19 and 32 i??m, respectively. When the FPF of SX was correlated with the concentration of FL in specific fractions, there was a marked, linear increase in FPF for increasing concentrations of FL in the 5-10 i??m fraction; however, all other fractions showed no significant increase in FPF. The study reflects the importance of lactose polydispersity in drug dispersion. Specific size fractions of cohesive FL enhance dispersion, while non-cohesive fractions of FL act as secondary carriers and decrease dispersion performance.