Allopurinol is a parasitistatic drug used for long time periods (≥6 months) in the treatment of canine leishmaniasis. Reports indicate that prolonged allopurinol therapy is associated with xanthinuria and xanthine urolithiasis. The purpose of this retrospective study was to describe the most common urinary adverse effects associated with allopurinol use in the treatment of canine leishmaniasis. Medical records of 320 dogs diagnosed with leishmaniasis in endemic areas were reviewed. All dogs received anti-Leishmania spp treatment with meglumine antimoniate once or twice daily for 4 weeks and allopurinol twice daily. Median duration of treatment with allopurinol until diagnosis of xanthinuria, renal mineralization, and/or urolithiasis was one year (range, 3 weeks to 9 years). Forty-two dogs (13.1%) developed adverse urinary effects defined by presence of xanthinuria: 9 of the 42 dogs (21.4%) developed xanthinuria alone; 9 (21.4%) had xanthinuria with urolithiasis; 11 (26.2%) showed xanthinuria with renal mineralization; and 13 (31%) developed xanthinuria, renal mineralization, and urolithiasis. Clinical signs associated with the urinary tract (eg, urinary obstruction, dysuria) developed in 19 of the 42 dogs (45.2%). No other adverse effects associated with allopurinol were reported. The authors concluded that xanthine urolithiasis and renal mineralization can occur in dogs secondary to allopurinol therapy, warranting monitoring for development of urinary adverse effects from the beginning of treatment.