We study the relation between the growth of a subharmonic function in the half space and the size of its asymptotic set. A function f defined on a domain D has an asymptotic value b∈[−∞, ∞] at a∈∂D if there exists a path γ in D ending at a such that u(p) tends to b as p tends to a along γ. The set of all points on ∂D at which f has an asymptotic value b is denoted by A(f, b). G. R. MacLane [10, 11] studied the class of analytic functions in the unit disk having asymptotic values at a dense subset of the unit circle. Hornblower [8, 9] studied the analogous class for subharmonic functions. Many theorems have since been proved having the following character: for a function f of a given growth, if A(f, +∞) is a small set then f has nice boundary behavior on a large set. See [1, 3–7] and the references therein. For α>0, let Mα be the class of subharmonic functions u in R + n + 1 ≡{(x, y):x∈ℝn, y>0} satisfying the growth condition u(x, y) ⩽ C(u)y−α for 0 < y < 1 for some constant C(u) depending on u. Denote by F(u) the Fatou set of u, which consists of points on ∂ R + n + 1 where u has finite vertical limits. For β>0, denote by Hβ the β-dimensional Hausdorff content. The following theorem is due to Barth and Rippon [1], Fernández, Heinonen and Llorente [5], and Gardiner [6].