Background/Aims: Jejunal biopsy with direct quantitative analysis of mucosal enzyme activities is the standard diagnostic method of lactase deficiency. But this test is seldom used in the clinical setting, because it is incovenient and cumbersome. On the contrary, sucrase to lactase activities ratio in duodenal mucosa is easy to measure and comp]iant with patients. We performed this study to investigate the value of sucrase to lactase activities ratio in duodenal mucosa as a diagnostic method of lactase deficiency. Methods: The 36 cases(22 men, 14 women) of this study were selected from healtby adults free from gastrointestinal symptom. We performed H breath test and syptom analysis after lactose load. And then, we obtained the duodenal mucosa by endoscopic biopsy and measured enzyme activities of the duodenal mucosa. Results: In 36 subjects, there were 21 subjects(58%) who had the intolerant symptoms and had more than a 20ppni increase in breath hydrogen. Sucrase to lactase activities ratio of the 21 subjects was more than 8. By the definition of sucrase to lactase activities ratio, the prevalence of lactase deficiency was 86 percent. This result was sirnilar to the previous prevalence(84%) defined by jejunal mucosa enzyme assay in Korean adults. Conclusion: Sucrase to lactase activities ratio of duodenal mucosa is a useful diagnostic tool for lactase deficiency. (Korean J Gastroenterol 1997;29:175 -181)