Background: Phylogenetic and genetic analyses have proven a valuable tool to infer epidemiological links between human immunodeficiency virus type-1 (HIV-1) isolates. These methods were applied in the present report for studying the genetic relatedness of the viral strains involved in two episodes of suspected HIV-1 transmission. Objectives: Provide any evidence that may help establish or refute the transmission link. Study design: In the first case, a leukemic patient became HIV-1 positive following the transfusion of platelets from a donor who was subsequently found to have tested false HIV-seronegative and to be sexual partner to an infected woman. In the second, a wife claimed to have acquired the infection from her husband who had concealed his infected status. Results and conclusions: The viral pairs detected in each of the suspected transmission cases exhibited common amino acid signatures and low genetic distances and segregated together in phylogenetic trees, thus showing a level of genetic relatedness similar to reference pairs known with certainty to be epidemiologically linked. These findings corroborated the existence of a direct transmission link in both the episodes with a high level of confidence. (C) 2003 Elsevier B.V. All rights reserved.