MicroRNA molecules consisting of 19-23 nucleotides influence numerous basic physiological and pathophysiological processes as endogenous mediators of RNA interference. These molecules are capable of specifically inhibiting the translation of messenger RNA molecules, but in some cases also promote the degradation of mRNA-s. Altered microRNA expression profiles were noted in several human diseases, most data, however, are known for neoplasms. Characteristic microRNA profiles are known both in solid and haematologic malignancies. MicroRNA profiles enable the distinction of benign follicular adenomas from follicular neoplasms of the thyroid. The micro-RNA expression patterns could be associated with the clinical behaviour of certain neoplasms (e.g. lung tumours and chronic lymphocytic leukemia) as well. It is possible that small molecular weight RNA-s may be used for therapeutical purposes in the future.