Described herein are micronized powder particles of colistimethate sodium wherein at least 50% by volume of the micronized particles have a diameter of less than 7 micrometers but not less than 3 micrometers and the powder has a total moisture content of from 5 to 10% by weight, for use in the treatment of a pulmonary infection by powder inhalation, wherein the colistimethate sodium is not separated into component form. The micronized powder particles of colistimethate sodium are useful in the treatment of infections caused by gram-negative bacteria, particularly in patients suffering from cystic fibrosis.