Transmission of human immunodeficiency virus type 1 (HIV-1) from a male accused of rape and deliberate transmission of HIV-1 was investigated by sequencing of the HIV-1 pol and gag genes from virus obtained from the male and from the female victim. Parts of the reverse transcriptase and p17gag genes were amplified and directly sequenced from uncultured peripheral blood mononuclear cells. The sequences were compared with sequences from 21 unrelated HIV-1-infected controls from the same geographic area (Stockholm, Sweden). Bootstrap analysis of phylogenetic trees demonstrated that the sequences from the female were significantly more closely related to the sequences from the male than to sequences from the controls. Furthermore, we found that the male and female shared two distinct genetic variants of HIV-1. In p17gag the major variant had an unusual, out-of-frame deletion of 3 nucleotides which the minor variant lacked. These results indicated that the male had transmitted more than one infectious unit to the female. From this study we concluded that it was highly likely that the HIV-1 strains carried by the male and female were closely epidemiologically linked.