Peutz–Jeghers syndrome (PJS) is a dominantly inherited human disorder characterized by gastrointestinal hamartomatous polyposis and mucocutaneous melanin pigmentation. LKB1 (STK11) serine/threonine kinase is the product of the causative gene of PJS, which has been mapped to chromosome 19p13.3. However, several studies have produced results that are not consistent with a link between LKB1 gene mutation and PJS. We constructed a knockout gene mutation of Lkb1 to determine whether it is the causative gene of PJS and to examine the biological role of the Lkb1 gene. Lkb1^−/− mice died in utero between 8.5 and 9.5 days postcoitum. At 9.0 days postcoitum, Lkb1^−/− embryos were generally smaller than their age-matched littermates, showed developmental retardation, and did not undergo embryonic turning. Multiple gastric adenomatous polyps were observed in 10- to 14-month-old Lkb1^+/− mice. Our results indicate that functional Lkb1 is required for normal embryogenesis and that it is related to tumor development. The Lkb1^+/− mouse is suitable for studying molecular mechanism underlying the development of inherited gastric tumors in PJS.