Cowden syndrome is very rare clinical condition with characteristic mucocutaneous lesions associated with abnormalities of the breast, thyroid, and gastrointestinal tract. Here, the case of a 16-year-old girl with diverse gastrointestinal polyposis with breast and thyroid lesions is reported as a definite case of Cowden syndrome. During follow up for 4 years, changes in the characteristic lesions were observed; Esophageal acanthosis and oropharyngeal polyps were newly developed. Gastric, duodenal, rectal and ileal polyps were noted at the first visit when she was 12 years of age and revealed histologically hyperplastic polyps. Mucocutaneous lesions, the pathognomonic finding of Cowden syndrome, were not noted at the first visit or during the follow up period. Breast and thyroid masses were noted at the first visit. Breast aspiration showed highly cellular ductal epithelial fragments and fibroblastic stromal cell fragments, and a thyroid biopsy shows proliferation of irregular sized follicles, with variable colloid contents. Macrocephaly was also noted at the first visit. The characteristic lesions of Cowden syndrome change with the patient's age, and the long-term follow up is recommended in cases suspected with this syndrome.