Our study aims to provide a comprehensive view of the endocrine features in Kennedy's disease (KD). Twenty-two men with KD underwent detailed endocrine investigations. Clinical signs of partial androgen resistance were present in more than 80% of the patients, with gynecomastia being the most prominent. Gynecomastia was postpubertal but appeared before muscular weakness in most cases. Thirteen patients had alteration of testicular exocrine function. Hormonal profile of partial androgen resistance was present in 86% of the patients, with an elevated testosterone level in 68%. Androgen insensitivity seems to appear later in life in KD, similar to the development of neurological signs. Although we confirm the previously reported correlation between the CAG repeat length and the early onset of the neurological disease, we describe a significant correlation between repeat length and the age of onset of gynecomastia as well as biological indexes of androgen insensitivity. This is supported by numerous in vitro data correlating variations in the CAG tract with androgen receptor activity; the longer the CAG repeats, the weaker the receptor transactivation. Ours is the first study to show such a clear and prominent pattern of androgen insensitivity in KD. In clinical practice, KD patients are often misdiagnosed as having amyotrophic lateral sclerosis. Careful examination of the endocrine component could avoid such a deleterious misdiagnosis.