Objective: We aimed to evaluate the effect on renal functions and blood pressure of the addition of low dose aspirin (LDA) to cyclooxygenase-2 (COX-2) inhibitors or nonselective NSAIDs. Methods: Two groups each containing 14 patients with one group using celecoxib and the other indomethacin regularly for at least 1 week were included into the study. Both groups were initially administered 100 mg/day (week 1), and later 300 mg/day (week 2) aspirin. Baseline and weekly serum creatinine, uric acid, electrolytes, creatinine clearance (CrCl) and blood pressure were obtained. Results: Contrary to the celecoxib group, in the indomethacin group, both after the first and the second weeks the mean serum creatinine increased and CrCl decreased when compared to baseline values (p values<0.05). In the indomethacin group, when compared to baseline values systolic blood pressure was significantly higher after week 1 and uric acid level after week 2 (p values=0.01). The frequency of patients with a ≥20% decrease in CrCl at the end of week 2 was higher in the indomethacin group than in the celecoxib group (42.9% vs. 0%, p=0.016). The difference between the mean creatinine (p=0.017) and CrCl values (p=0.007) from baseline until after week 2 was more significant in the indomethacin group than in the celecoxib group. Conclusions: The addition of LDA to patients using indomethacin led to significant renal dysfunction. Subjects using celecoxib seem to have been protected from the renal side effects of LDA.