A 9-year-old girl was brought for consultation due to autism and a large head circumference. Her birth weight was 6 pounds after a 37-week gestation to a healthy G3P1SAb 2 mother. She had been a healthy child with the exception of the development of a lipomatous lesion on the left thigh, requiring surgical removal at age 3(1/2) years. Autism was diagnosed at age 5 yr by a developmental pediatrician. She did not have cognitive disabilities or a history of seizures. The family history was notable for maternal infertility with no history of developmental disabilities, large body or head size, or malignancy in close relatives.On physical examination, she was a mildly obese girl with a large head. Her weight was 50.4 kg (>95%), height was 142 cm (90%), and head circumference was 60.3 cm (significantly >95%; 4SDs above the mean). Examination of her skin was notable for a 2 x 6 cm scar on her left thigh and three café au lait macules on her trunk. She was Tanner Stage I. Mild hypotonia with normal deep tendon reflexes was observed; the remainder of the neurological examination was normal.Laboratory studies included high-resolution chromosomes, fragile X, metabolic screens, and methylation for Prader Willie Syndrome and Angelman Syndrome; all these studies were normal. Molecular testing of the PTEN gene (phosphatase and tensin homolog protein) revealed a R355X mutation, consistent with the diagnosis of Bannayan-Riley-Ruvalcaba Syndrome (BRRS). In parents and siblings, PTEN test results were negative for mutations.Endocrine evaluation revealed an abnormal thyroid nodule on ultrasound. Computed tomography and positron emission tomography scans raised suspicion of malignancy. She underwent a total thyroidectomy; the pathology report revealed a thyroid adenoma with Hurthle cells. She was treated with thyroid hormone replacement therapy.