N- and K-ras oncogene mutations represent the most frequent molecular lesions in plasma cell dyscrasias. They are not randomly distributed since they are detectable in multiple myeloma (MM) (9-31%) and plasma cell leukemia (PCL) (30%), and not in monoclonal gammopathy of undetermined significance (MGUS) and solitary plasmacytoma (SP). Codons 12, 13 and 61 of N- and K-ras genes have been found mutated. Mutations affecting codon 61 of N-ras gene are the most frequent finding. A heterogeneous pattern of mutations is described with a prevalence of purine-pyrimidine transversions. Ras gene mutations have been predominantly detected in myelomas characterized by an advanced stage disease, and adverse prognostic parameters. These findings suggest that ras mutations represent a late molecular lesion and may be implicated in tumor progression rather than tumor initiation.