RNA interference (RNAi) silences gene expression by guiding mRNA degradation in asequence-specific fashion. Small interfering RNA (siRNA), an intermediate of the RNAi pathway, has been shown to be very effective in inhibiting virus infection in mammalian cells and cultured plant cells. Here, we report that Agrobacterium tumefaciens-mediated transient expression of short hairpin RNA (shRNA) could inhibit tobacco mosaic virus (TMV) RNA accumulation by targeting the gene encoding the replication-associated 126 kDa protein in intact plant tissue. Our results indicate that transiently expressed shRNA efficiently interfered with TMV infection. The interference observed is sequence-specific, and time- and site-dependent.Transiently expressed shRNA corresponding to the TMV 126 kDa protein gene did not inhibit cucumber mosaic virus (CMV), an unrelated tobamovirus. In order to interfere with TMV accumulation in tobacco leaves, it is essential for the shRNA constructs to be infiltrated into the same leaves as TMV inoculation. Our results support the view that RNAi opens the door for novel therapeutic procedures against virus diseases.We propose that a combination of the RNAi technique and Agrobacterium-mediated transient expression could be employed as a potent antiviral treatment in plants.