The effect of the spacers (Fisonair®, Breath-A-Tech®, Volumatic® and Nebuhaler®) on the in vitro aerosol characteristics of two propellant-driven metered dose inhalers (MDIs), Tilade® (nedocromil sodium) and Intal® (disodium cromoglycate), was studied. The measurement was carried out on a Marple–Miller impactor operating at 30 l/min. Five actuations were collected for the drug assay. The results showed that Tilade® (label claim 2 mg active per actuation) and Intal® (label claim 5 mg active per actuation) generated aerosols with a fine particle mass (FPM, i.e. mass of particles 5 μm in the aerosol) of 0.34 mg (S.D. 0.01, n=4) and 0.02 mg (S.D. 0.01, n=4) per actuation, respectively. For both inhalers, large volume spacers increased (Fisonair®>Nebuhaler®>Volumatic®) while small volume spacer (Breath-A-Tech®) decreased the FPM. The FPM (per actuation) for Tilade® with Fisonair®, Nebuhaler®, Volumatic® and Breath-A-Tech® was 0.52 (0.03), 0.45 (0.03), 0.41 (0.04) and 0.09 (0.04) mg, respectively, while for Intal® the corresponding values were 0.41 (0.02), 0.32 (0.04), 0.28 (0.03) and 0.08 (0.01) mg. Thus, the fine particle mass can be either increased or decreased, depending on the spacer selected. In addition, all spacers significantly reduced the coarse particle (≥10 μm) mass, with Fisonair®, Breath-A-Tech®, Nebuhaler® and Volumatic® producing only 7.6, 0.4, 5.2 and 2.6%, respectively of that from Tilade® alone and 15.6, 0.7, 5.4 and 4.1%, respectively of that from Intal® alone. The general trends for Tilade® and Intal® were similar but not quantitatively identical. The proper choice of spacers is therefore important for the optimal delivery of Tilade® and Intal®.