The present invention provides a method for producing particles having a reduced particle size distribution, which method comprises the steps of: a) providing a composition comprising particles; b) subjecting the particles in said composition to a size reduction step or to a size growth step; c) feeding said particles to a first membrane separation system to separate said particles according to size; d) recycling those particles that do not meet the size criteria back to step a); e) optionally, collecting in a collection tank the permeate of the first membrane separation system. Particles obtainable according to the method of the invention and characterized by having a near monodisperse particle size distribution are also provided. The particles are preferably characterized by having a particle size distribution with a span of less than 2.0. The invention also provides pharmaceutical compositions comprising particles according to the invention, and also apparatus for carrying out the method of the invention.