We present the first known case of somatic PTEN mosaicism causing features of Cowden syndrome (CS) and inheritance in the subsequent generation. A 20-year-old woman presented for genetics evaluation with multiple ganglioneuromas of the colon. On examination, she was found to have a thyroid goiter, macrocephaly, and tongue papules, all suggestive of CS. However, her reported family history was not suspicious for CS. A deleterious PTEN mutation was identified in blood lymphocytes, 966A>G, 967delA. Genetic testing was recommended for her parents. Her 48-year-old father was referred for evaluation and was found to have macrocephaly and a history of Hashimoto's thyroiditis, but no other features of CS. Site-specific genetic testing carried out on blood lymphocytes showed mosaicism for the same PTEN mutation identified in his daughter. Identifying PTEN mosaicism in the proband's father had significant implications for the risk assessment/genetic testing plan for the rest of his family. His result also provides impetus for somatic mosaicism in a parent to be considered when a de novo PTEN mutation is suspected.