The present concepts of amyotonia congenita are briefly outlined. Particular reference is made to the clinical syndrome of benign congenital hypotonia, which corresponds to Oppenheim's original concept of a sporadic benign condition and represents the cases of amyotonia not due to infantile progressive muscular atrophy on symptomatic of any other disease entity. The case of a boy is reported who has such a benign form of hypotonia, as well as undescended testes, delayed osseous development and a few other congenital stigmata. He has been studied from birth to the age of nearly 3 years and has been found to have an extra small chromosome resembling the Group 21-22 + Y. The diagnosis and findings are discussed, with particular reference to the possible significance of the chromosomal anomaly. Suggestions for the future investigation of similar cases are made.