Double-stranded RNA (dsRNA) induces the homology-dependent degradation of cognate mRNA in the cytoplasm via RNA interference (RNAi), but also is a target for adenosine-to-inosine (A-to-I) RNA editing by adenosine deaminases acting on RNA (ADARs). An interaction between the RNAi and the RNA editing pathways in C. elegans has been suggested recently, but the precise mode of interaction remains to be established. In addition, it is unclear whether this interaction is possible in mammalian cells with their somewhat different RNAi pathway. Here we show that ADAR1 and ADAR2, but not ADAR3, avidly bind short-interfering RNA (siRNA) without RNA editing. Especially, the cytoplasmic full-length isoform of ADAR1 has the highest affinity among known ADARs, with a sub-nanomolar dissociation constant. Gene silencing by siRNA is significantly more effective in mouse fibroblasts homozygous for an ADAR1 null mutation than in wild-type cells. In addition, suppression of RNAi effects are detected in fibroblast cells over-expressing functional ADAR1, but not when over-expressing mutant ADAR1 lacking dsRNA binding domains. These results identify ADAR1 as a cellular factor that limits the efficacy of siRNA in mammalian cells.