Let k be an infinite field. We fully describe when the unit group of a semigroup algebra k[S] of a semigroup S generated by finitely many periodic elements satisfies a group identity. This and some other recent results are proved by first showing that semiprime k-algebras generated by units are necessarily reduced whenever their unit group satisfies a group identity. 2004 Elsevier Inc. All rights reserved.The unit group U(R) of a ring R with unity 1 is said to satisfy a group identity (we call R a GI-ring for short) if there exists a non-trivial word w(x 1 , . . . , x n ) in the free group generated by x 1 , . . . , x n such that w(u 1 , . . . , u n ) = 1 for all u 1 , . . . , u n ∈ U(R).We start with a brief overview of some recent results on this topic. As a consequence of a more general result, Valitskas in [23] proved that if A is an algebra over an infinite field ✩ Research partially supported by the Onderzoeksraad of Vrije Universiteit Brussel and Fonds voor Wetenschappelijk Onderzoek (Vlaanderen), Flemish-Polish bilateral agreement BIL 01/31, FAPESP-Brazil and CNPq-Brazil (Proc. 300652/95-0).