Proviral nucleotide sequences from the p7 region of the gag gene were compared among 40 Baltic and Russian HIV-1 strains. 33 of the infected persons carried a virus belonging to subtype B. Thus subtype B, which is most prevalent in Western Europe, was the most common subtype in all 3 Baltic countries (Estonia, Latvia and Lithuania) and Russia, and was associated with homosexual transmission. It also seemed that viruses of the same origin have been circulating in these countries; most of the subtype B viruses studied (n = 22) belonged to a closely related virus pool with average inter-nucleotide sequence distances of 2.7%. In 7 cases, viruses of non-B subtypes were found (1 subtype D from Estonia, 1 subtype C from Latvia, 1 subtype A from Lithuania and 1 subtype G from Russia). Three closely related viruses (1 from Lithuania and 2 from Russia) remained unclassified.