Abstract. We study stable solutions of the following nonlinear systemand Ω is a domain in R n . We introduce the novel notion of symmetric systems. The above system is said to be symmetric if the matrix of gradient of all components of H is symmetric. It seems that this concept is crucial to prove Liouville theorems, when Ω = R n , and regularity results, when Ω = B 1 , for stable solutions of the above system for a general nonlinearity H ∈ C 1 (R m ). Moreover, we provide an improvement for a linear Liouville theorem given in [20] that is a key tool to establish De Giorgi type results in lower dimensions for elliptic equations and systems.