OBJECTIVE To examine the expression of myc proto-oncogenes; c-myc, L-myc, and N-myc, and their related genes max and mad, in the arthritic synovium. METHODS Using reverse transcription-polymerase chain reaction (RT-PCR), Northern and Southern hybridizations, the expression of these genes in the synovial tissue from rheumatoid arthritis (RA) and osteoarthritis (OA) was analyzed. Synovial specimens from cadavers without any joint disease and peripheral blood mononuclear cells (PBMC) from healthy individuals served as controls. RESULTS As a novel finding, synovial cells were observed to express L-myc, N-myc as well as their related genes max and mad, in addition to the previously described presence of c-myc proto-oncogene in synovium. c-myc, L-myc, N-myc, and mad were expressed in all patient samples studied, including the controls. Instead, max was detected in only 10/12 of RA patients, in 11/13 of OA patients, and in all controls (4/4 cadavers, 5/5 blood donors). Six patients with RA revealed positive signals for max only after hybridization. The same was also true of two patients with OA and of one healthy individual donating blood. CONCLUSIONS The L-myc, N-myc, max, and mad genes are expressed in synovial cells, in addition to c-myc proto-oncogene. However, expression of these genes is not disease-specific, since they were equally expressed in synovial samples from patients with RA or OA as well as from cadavers representing controls without any joint disease.