Serum cytidine deaminase (CD) as a marker of inflammatory disease was assessed in 44 patients and 47 controls to differentiate polymyalgia rheumatica (PMR) from elderly onset rheumatoid arthritis (EORA). The patients were divided into four groups: PMR with and without synovitis and seropositive and seronegative EORA. No statistically significant differences were found when serum CD levels of seropositive EORA patients were compared with serum CD of PMR patients without synovitis, neither when serum CD levels of all PMR patients were compared with a seronegative EORA group, nor when serum CD levels of PMR patients with synovitis were compared with those with EORA. Nevertheless, statistically significant differences were detected between EORA's serum CD levels and the control group (p=0.023). This difference was 10% when comparing CD levels of PMR patients with the control group (p=0.070). We did not demonstrate that serum CD levels could be a useful tool to differentiate PMR from EORA, but these findings could nevertheless reflect the presence of an inflammatory disease.