Precocious puberty still remains an elusive diagnosis in the majority of patients. Infrequently, lesions of the central nervous system are associated with sexual precocity. Depending on their location, these cysts may affect many systems, however, there is little information concerning their involvement in endocrinological disorders. We report a case of a sylvian cistern arachnoid cyst presenting with precocious puberty in a 3-year-old girl. The child recovered following a cystoperitoneal shunt. The mass effect of the arachnoid cyst upon the hypothalamus was, at least in part, responsible for the development of precocious puberty. To the best of our knowledge, this is the 1st case of a sylvian cistern arachnoid cyst presenting with precocious puberty. The role of surgical decompression of the cyst is also discussed.