The article is concerned with the Bourgain, Brezis and Mironescu theorem on the asymptotic behaviour of the norm of the Sobolev-type embedding operator: Ws,p ? Lpn/(n-sp) as s ? 1 and s ? n/p. Their result is extended to all values of s ? (0, 1) and is supplied with an elementary proof. The relation is proved. © 2002 Elsevier Science (USA).