OBJECTIVES:  To estimate the effect of allopurinol on hyperuricemia and estimated glomerular filtration rate (eGFR). METHODS:  An observational non-concurrent prospective cohort study. Patients older than 50 years with hyperuricemia were included. All patients received allopurinol 100-300 mg day for 12 months. The levels of uric acid (UA) were determined and the glomerular filtration rate (GFR) was estimated at baseline. Changes in AU and in GFR were observed after treatment. RESULTS:  50 patients diagnosed with hyperuricemia on treatment with allopurinol were eligible with a mean age of 71.6 + 11.1 years. 58% of the patients were male and 46% had diabetes mellitus. UA levels decreased significantly from 7.1 ± 1.1 mg / dl to 5.5 ± 1.2 mg / dl (p <0.001). The GFR increased from 46.3 ± 19.1 ml / min / 1.73 m2 to 49.0 ± 20.6 ml / min / 1.73 m2 but this increase was not significant (p 0.14). There were no reports of adverse reactions to allopurinol. CONCLUSIONS:  In this study, allopurinol was effective and safe in the treatment of hyperuricemia but did not significantly increase GFR in patients older than 50 years.