Down-up algebras A = A(alpha, beta, gamma) were introduced by G. Benkart an
d T. Roby to better understand the structure of certain posets. In this pap
er, we prove that beta not equal 0 is equivalent to A being right (or left)
Noetherian, and also to A being a domain. Furthermore, when this occurs, w
e show that A is Auslander-regular and has global dimension 3.