In many respiratory infections caused by multi-drug-resistant Gram-negative bacteria, colistin is often the last-line drug for treatment despite its nephrotoxicity when administered parenterally. Inhalation therapy of colistin has great potential to improve the efficacy while reducing adverse effects. In this study, inhalable powder formulations of colistin (sulphate) were produced via spray drying. The colistin powders were found to have intact antimicrobial activity against Acinetobacter baumannii measured by broth micro-dilution. Both the raw material and spray-dried formulations were amorphous and absorbed significant amount of water up to 30 (w/w) at relative humidity (RH) of at least 70 . The spray-dried formulations were physically stable in the amorphous form at 60 RH and 25?C, having a high aerosol efficiency (emitted dose >86 and fine particle fraction total >83 ) which remained unchanged after a 3-month storage. Storage at an elevated RH of 75 resulted in the aerosolisation performance significantly decreased, and at RH 90 , the formulation particles fused together (but without re-crystallisation). Although spray drying has been extensively used for generating inhalable drug particles, this is the first report that colistin powder can be physically stable in the amorphous form at ambient conditions, indicating that spray-drying approach is suitable for producing inhalable colistin powder formulation.