Inhaled pentamidine has become an important method of treatment and prophylaxis for Pneumocystis carinii pneumonia and we have compared nebuliser efficiency in terms of drug output and droplet sizes in four brands of jet nebuliser (Acorn-22, Inspiron, Cirrus, Respirgard II) and one brand of ultrasonic nebuliser (Fisoneb), at 2 pentamidine concentrations and 3 flow rates, using a laser particle sizer. Droplet size (which varied from 1.2 to 4.7 microns mass median diameter) was dependent in all cases, except with the Respirgard II system, on the flow rate of the gas driving the equipment and the concentration of pentamidine used. Drug output varied significantly between nebuliser brands and for a 300 mg dose of pentamidine was: 61% for the Acorn-22, 62% for the Inspiron, 49% for the Fisoneb and 43% for the Respirgard II. Both droplet size and drug output are important in determining nebuliser efficiency.