BACKGROUND: There is accumulating evidence on the role of pesticides in the etiology of pemphigus vulgaris (PV). OBJECTIVE: To determine whether chlorpyrifos, an organophosphate pesticide, is involved in the immunopathology of PV. METHODS: Normal human skin biopsy specimens incubated with progressively diluted chlorpyrifos solutions were used as indirect immunofluorescence substrates for sera from two PV patients previously exposed to the agent and from healthy controls. Involvement of T-cell lymphocytes was assessed by release of interferon-g in the presence of chlorpyrifos. RESULTS: In one PV patient, immunofluorescence was strongly positive for the specimen incubated with the pesticide and weakly positive for the specimen incubated with medium alone. Immunofluorescence was negative in the patient under immunosuppression with prednisolone and in all controls. Both patients tested positive on interferon-g assay; controls tested negative. CONCLUSIONS: Findings suggest an immunopathogenic role of chlorpyrifos in PV. Interferon-g cytokine assay with the pesticide combined with immunofluorescence tests may provide an in vitro diagnostic tool in suspected pesticide-induced/exacerbated pemphigus.