OBJECTIVE Biliary cholesterol supersaturation, rapid nucleation of cholesterol, and altered gallbladder motility are prerequisite for gallstone formation. However, the pathogenesis of microlithiasis is not clear. The aim of this study was to determine the abnormalities of gallbladder emptying and bile composition in patients with microlithiasis. METHODS Nucleation time, cholesterol saturation index (CSI), and gallbladder emptying were studied in patients with microlithiasis (n = 10), patients with gallstones (n = 10), and healthy volunteers (n = 10). Bile analysis was repeated in 6 patients with microlithiasis treated with ursodeoxycholic acid (UDCA) for 8 weeks. RESULTS Nucleation time was shorter in patients with microlithiasis and those with gallstones than in healthy volunteers (P < 0.0001). Patients with microlithiasis had longer nucleation time than those with gallstones (P < 0.001). There was no difference in cholesterol levels and CSI in gallstone and microlithiasis patients. However, healthy volunteers had lower cholesterol levels (P < 0.01) and CSI (P < 0.01). Patients with microlithiasis had prolongation of nucleation time (P < 0.001) and lowering of CSI (P < 0.001) after UDCA therapy. Gallbladder ejection fraction was higher in microlithiasis patients than in gallstone patients (P < 0.01) but lower than in healthy volunteers (P < 0.01). CONCLUSIONS Patients with microlithiasis have longer nucleation time and better gallbladder emptying than patients with gallstones. Bile abnormalities can be successfully corrected with UDCA therapy in patients with microlithiasis.