Since stone formers are advised to increase their intake of fluid, the present study was undertaken to determine the effect of cola beverage consumption on calcium oxalate kidney stone risk factors. Fourteen males and 31 females provided 24-h urines before and after an acute load of cola. Relative supersaturations, activity products and empirical risk indices, ratios and quotients were calculated from urinary biochemical data to assess calcium oxalate crystal and stone formation risk. Several risk factors changed unfavourably following consumption of cola. In males, oxalate excretion, the Tiselius risk index and modified activity product increased significantly (P < 0.05). In females, oxalate excretion increased significantly while magnesium excretion and pH decreased significantly (P < 0.05). Scanning electron microscopy showed that urines obtained from both sexes after cola consumption supported calcium oxalate crystallization to a greater extent than the control urines. It is concluded that consumption of cola causes unfavourable changes in the risk factors associated with calcium oxalate stone formation and that therefore patients should possibly avoid this soft drink in their efforts to increase their fluid intake.