The recently cloned STK11/LKB1 on chromosome 19p has been shown to be a new tumor suppressor gene. Mutations in the LKB1/STK11 gene on chromosome 19p account for most cases of Peutz–Jeghers syndrome (PJS), in which intestinal hamartomas are associated with elevated risks of several cancer types, including breast cancer. A previous study revealed that familial breast cancer is associated with loss of heterozygosity (LOH) on 19p. To establish whether germline mutations of STK11/LKB1 account for familial breast cancer, 22 patients from 14 breast cancer families with LOH on 19p and one PJS family were selected for screening for germline mutations of LKB1/STK11. A combination of polymerase chain reaction (PCR)-heteroduplex, single-strand conformational polymorphism (SSCP) analyses, Southern blot analysis and direct sequencing were used for mutation detection. No mutations were identified. Germline mutations of LKB1/STK11 did not contribute to breast cancer in these families.