Hollow polymer microcapsules are made by the method of dissolving a film-forming polymer in a volatile nonaqueous solvent; dispersing into the polymer solution finely divided particles of a volatilizable solid core material; inducing formation of a solid polymer coating on the particulate solid core material in the nonaqueous liquid mixture to produce polymer microcapsules having an encapsulated core of particulate core material; recovering the polymer microcapsules from the nonaqueous liquid mixture; and removing the encapsulated core material from the microcapsules to make hollow polymer microcapsules. Gas-filled polymer microcapsules that are made according to the method of this invention are useful in medical applications such as imaging contrast agents because they may be prepared to precisely controlled size specifications.