Abstract. In this note, we show that when G is a torsion group the second center of the group of units U (ZG) of the integral group ring ZG is generated by its torsion subgroup and by the center of U (ZG). This extends a result of from finite groups to torsion groups, and completes the characterization of hypercentral units in ZG when G is a torsion group.Let ZG denote the integral group ring of a torsion group G, U (ZG) the group of units of such a group ring and V (ZG) the subgroup of units of augmentation 1.In the case where G is finite, the ascending central serieswhere T denotes the torsion subgroup of Z 2 (V (ZG)). The first of these results was extended to torsion groups in [5], and the question of whether or not the second result could be similary extended was Open Problem 5 in [8].In this note, we show that the second result can indeed be extended to torsion groups. Although Blackburn's theorem [3] on the intersection of nonnormal subgroups of finite groups played a significant role in the investigation of the structure of the second center in [2], this result cannot be applied directly to our case. Instead, our argument focusses on the importance of Bass cyclic, bicyclic and Hoechsmann units in integral group rings.Notation and terminology will follow that in [10]. The following lemma is crucial to our approach. Although the argument given can be found in [5] (as part of the proof of Theorem 2), we include it here for completeness.Since cg is of finite order, it follows that c is also of finite order and hence is in G [9, p.462 is not contained in Z 1 (V (ZG)), then there must exist a group element h in Z 2 (V (ZG))\Z 1 (V (ZG)). But then if u is any unit in ZG, [u, h] = h 0 ∈ Z 1 (G) and there exists a positive integer n = n(u) such that u n hu −n = h. It follows from Theorem 1.2 of [7] that the exponent of Z 1 (G) is 2, and so for all u 2 ∈ Z 2 (V (ZG))