Increase in body size increases the risk of renal stone formation. The mechanism explaining this relationship remains unclear. Urine pH is one of the important factors for urinary stone formation. The purpose of this study was to determine whether there is an association between urine pH and body mass index (BMI) in patients with urolithiasis. Medical charts review that included 342 urinary stone formers (248 men and 94 women). Data obtained included patient sex, age, BMI, urine pH at diagnosis, and stone composition. The patients were classified as normal weight (18.5 <or= BMI < 24), overweight (24 <or= BMI < 27), or obese (BMI >or= 27). The mean urine pH of the normal body weight, overweight, and obese groups was 6.25, 6.14, and 6.00, respectively (P < 0.05). Urine pH is inversely related to BMI among patients with urolithiasis. Among patients with urolithiasis, higher BMI will have lower urine pH. This may explain why obesity is associated with an increased risk of nephrolithiasis. Weight loss should be explored as a potential treatment to prevent kidney stone formation. The prevention of urinary stone disease gives clinicians an additional reason to encourage weight reduction through diet.