A process for the preparation of entacapone, in particular as the polymorphic form A, comprising the preparation of a compound of formula (V), wherein R is C1-C6 alkyl, by condensation of N,N-diethyl-cyano-acetamide with a compound of formula (IV), wherein R is C1-C6 alkyl, in the presence of a strong basic agent; the dealkylation of the compound of formula (V) to obtain entacapone, and the crystallization thereof from methyl ethyl ketone is performed to yield the polymorphic form A. The polymorphic form A of entacapone may be used to treat Parkinson's disease and/or to enhance effectiveness of muscle control.