Gall-stone dissolution rate was measured in 52 patients with radiolucent gall stones in a functioning gall bladder receiving chenic acid 15 mg/kg/day. Percentage reduction in gall-stone area at six months was inversely related to initial gall-stone diameter (n = 52; r = -0.53; p less than 0.001), and to cholesterol saturation index of fasting gall-bladder bile during treatment (n = 28; r = -0.61; p less than 0.001). The duration of treatment required for complete dissolution was directly related to initial gall-stone diameter (r = 0.47; p less than 0.02). We conclude that the rate of gall-stone dissolution depends not only on gall-stone size but also on the degree of unsaturation of gall-bladder bile achieved during chenic acid treatment.