Xanthine calculi were found in a 7‐month‐old male Cavalier King Charles spaniel with urethral obstruction and renal insufficiency. Because the only two other reported cases of naturally occurring xanthine urolithiasis concerned a Cavalier King Charles and a King Charles spaniel the urine of the littermates and parents of the patient were also examined for xanthinuria. Semi‐quantitative analysis revealed high urine concentrations of hypoxanthine and xanthine in the patient and his female littermate. Quantitative analysis by high‐pressure liquid chromatography (HPLC) of the urine samples from the family of this Cavalier King Charles spaniel and nine control dogs revealed that hypoxanthine and xanthine excretion was 30 and 60 times higher in the affected patient and the female littermate than in the others dogs. The pattern of xanthinuria, which is caused by a deficiency of the enzyme xanthine oxidase, in the relation diagram of this family of Cavalier King Charles Spaniels was consistent with an autosomal recessive mode of inheritance.