Peutz-Jeghers syndrome (PJS) is a rare autosomal dominant inherited disorder characterized by gastrointestinal polyposis and mucocutaneous pigmentation. PJS patients have an increased risk of cancer in multiple locations. Germ-line mutations in the STK11 gene have been found to be responsible for most PJS cases. DNA samples were obtained from a Chinese child with PJS, his clinically unaffected parents and 50 unrelated normal individuals, and the exons and flanking intronic sequences of the STK11 gene were analysed by polymerase chain reaction and direct sequencing. A novel de novo mutation (c.698_699insG; F234LfsX3) was identified in exon 5 of STK11, that resulted in a translational frameshift leading to termination at codon 236. This mutation was not found in the parents or unrelated individuals. These results enlarge the genotypic spectrum of STK11, particularly with regard to early onset, as observed in the present sporadic PJS case. This study may have important future implications for precise genotype-phenotype correlation research.