Effective liver blood flow is the portion of total flow that perfuses functional sinusoids and is available for metabolic exchange. Clearance of galactose from blood at concentrations below 10 mg/dl (0.555 mmol/l) measures this index and is calculated during continuous infusion of 5% D-galactose at a rate of 50 mg/min. The low galactose concentrations are measured accurately by a new fluorometric assay, which gives a precision +/- 0.2 mg/dl (0.011 mmol/l). In healthy people, plasma galactose clearance was 1366 +/- 172 ml/min, and hepatic extraction was 95%. Clearance in cirrhotics depends on the stage of their disease: in a stable group of patients with advanced cirrhosis, clearance was 835 +/- 87 ml/min with hepatic extraction ranging from 60% to 95%. The day-to-day coefficient of variation was 4.5%. Direct comparison with flow-probe liver blood flow measured in 11 normal dogs showed that galactose clearance was not significantly different. These findings support the hypothesis that galactose clearance correlates with effective liver blood flow.