Abstract: The present invention provides, among other things, an improved method of treating cystic fibrosis (CF) in a human subject. The method comprises administration of a composition comprising an mRNA encoding a Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) protein by nebulization at a dose between 7 mg and 25 mg. A suitable dose for use in the method of the invention is selected on the basis that it provides the human subject with at least a 3% increase in absolute change in ppFEV1 (percent predicted forced expiratory volume in one second) from baseline ppFEV1 at two days following the administration. In addition or alternatively, the dose is selected to provide the human subject with at least a 2% increase in absolute change in ppFEV1 from baseline ppFEV1 at one week following the administration.