Nucleation time (time elapsed until first appearance of cholesterol crystals in incubated bile fluid) and the lithogenic index were measured on 25 patients (aged 45.1 +/- 16.8 years; Broca index 106.7 +/- 4.2%) with radiolucent gallstones, before receiving daily 500 mg chenodeoxycholic acid and ursodeoxycholic acid orally. The nucleation time in the group of nine patients in whom complete lysis was achieved was 4.7 +/- 0.8 days, compared with 15.0 +/- 2.2 days in those with only incomplete lysis (P less than 0.001). There was no statistically significant difference between the two groups as to age, Broca index, lithogenic index and biochemical values. Thus the determination of nucleation time can contribute to distinguishing between responders and nonresponders to oral treatment for dissolving gallstones and may improve the success rate of the method.