To assess in vivo sequence heterogeneity of the human immunodeficiency virus type 1 (HIV-1) env gene, we used the polymerase chain reaction to amplify proviral sequences present in peripheral blood mononuclear cell DNA of a patient with acquired immunodeficiency syndrome (AIDS). The amplified env gene fragment (575 bp) contains the first hypervariable region and part of the first conserved region. Eleven and twelve clones were sequenced, respectively, from specimens collected two months apart. Notable heterogeneity was observed among sequences recovered from both specimens. Also, the proviral population recovered from the first specimen varied significantly from that found in the second specimen. Both specimens contained forms with and without an 18 bp duplication. The presence or absence of this duplication, in addition to several point mutations, appear to define two molecular groups evolving in parallel within this patient. Several genotypes which had sequences characteristic of both groups occurred primarily in the second specimen; these can best be explained by multiple recombinational events between representatives of the two groups during reverse transcription. This study demonstrates that recombination may contribute significantly to the generation of diversity among HIV variants within a single individual.