BACKGROUND: It has not been clearly demonstrated whether HIV vaccination can complicate routine HIV testing. In this report, we describe the laboratory data of two prisoners who received rgp120 vaccine in a phase III trial underway in Thailand. These data indicate that previous vaccination may complicate the interpretation of screening HIV diagnostic tests. CASE PRESENTATION: The participants were identified from a cohort study on "Health factors related to HIV-1 and other viral infections among incarcerated people" that was approved by The Ethical Committee for Research in Human Subjects, Ministry of Public Health, Thailand. HIV diagnosis was definitively established with serial specimens using multi-screening tests, Western blot and diagnostic PCR.Anti-HIV screening tests consistently exhibited either weakly reactive or inconclusive results. The band patterns of the Western blot analysis corresponded to those found in individuals who received the rgp120 vaccination. Definite results were established using diagnostic PCR, which exhibited consistently negative results with follow-up specimens. Such problems in HIV testing are not easily resolved in the routine clinical setting in Thailand. CONCLUSIONS: These data demonstrate that HIV-1 vaccination interferes with routine diagnostic tests. Similar cases will not be uncommon in Thailand, where 2,545 people have already participated in a phase III trial.