The aim of the study was to assess the influence of overweight and obesity on the risk of calcium oxalate stone formation. BMI, 24-hour urine, and serum parameters were evaluated in idiopathic calcium oxalate stone formers (363 men and 164 women) without medical or dietetic pretreatment. Overweight and obesity were present in 59.2% of the men and in 43.9% of the women in the study population. Multiple linear regression analysis revealed a significant positive relationship between BMI and urinary uric acid, sodium, ammonium, and phosphate excretion and an inverse correlation between BMI and urinary pH in both men and women, whereas BMI was associated with urinary oxalate excretion only among women and with urinary calcium excretion only among men. Serum uric acid and creatinine concentrations were correlated with BMI in both genders. Because no association was established between BMI and urinary volume, magnesium, and citrate excretion, inhibitors of calcium oxalate stone formation, the risk of stone formation increased significantly with increasing BMI among both men and women with urolithiasis (p = 0.015). The risk of calcium oxalate stone formation, median number of stone episodes, and frequency of diet-related diseases were highest in overweight and obese men. Overweight and obesity are strongly associated with an elevated risk of stone formation in both genders due to an increased urinary excretion of promoters but not inhibitors of calcium oxalate stone formation. Overweight and obese men are more prone to stone formation than overweight women.