Polymorph screening of formoterol fumarate was performed in 12 solvents, followed by evaluations of thermodynamic stability. Three anhydrates, a dihydrate, a diethanolate, a diisopropanolate, and a dibensylalcoholate were found. The crystal structure of three solvated modifications and of the most stable anhydrate was investigated. This indicated that solvation is needed to get a stable and well packed crystal structure. Thermodynamic testing suggests that five crystal modifications are thermodynamically stable, at different conditions, since they are all reversibly related to each other.