Consider the following polyharmonic equations: (−) r u = f (u) in O, (0.1) where O = R n or O = R n + with the Dirichlet boundary conditions, r ∈ N * and n ≥ 2r + 1. We prove some Liouville-type theorems classifying stable (or stable at infinity) solutions, possibly unbounded and sign-changing. Regarding the class of stable solutions, we focus on the case of superlinear nonlinearities f with subcritical or critical growth near zero, like f (s) = |s| p−1 s(1 + c 0 |s| q) or f (s) = |s| p−1 s exp(s 2), where 1 < p ≤ n + 2r n − 2r , q > 0 and c 0 ≥ 0. Our approach to get the main integral estimates makes use of delicate analysis with appropriate test functions and weighted seminorms. We also establish a variant of Pohozaev identity (Pohozaev in Sov Math Dokl 5:1408-1411, 1965). This permits us to get classification result for stable at infinity solutions under the global subcritical condition: 2n n − 2r F(s) − f (s)s > 0, ∀s = 0, where F(s) = s 0 f (t)dt. Our assumptions can be verified by many nonlinearities very close to the critical growth, like f (s) = c|s| p−1 s + |s| 4r n−2r s ln q (s 2 + a) , where 1 < p < n+2r n−2r , q ≥ 0, c ≥ 0 and a > 1, with 4r n−2r − 2q a ln(a) > 0; or c > 0 if q = 0.