Background: Increased sensitivity to proactive (PI) and retroactive (RI) interference has been observed in amnestic mild cognitive impairment (aMCI). PI and RI are often explained as being the result of a response competition mechanism. However, patients with aMCI are supposed to suffer mostly from encoding deficits. We hypothesized that in aMCI interference may occur at encoding and not only at the retrieval stage. Material and methods: We developed an original paradigm enabling PI and RI to be tested with and without response competitors. Eighteen young controls (YC), 16 elderly controls (EC) and 15 aMCI participated in the study. Results: The YC and EC groups presented interference effects only in conditions that included a direct response competitor. In contrast, aMCI had interference effects in all conditions including the one without response competitor. Conclusion: Increased sensitivity to interference in aMCI appears to occur at the encoding/consolidation stage and not only at the retrieval stage, as is the case in healthy subjects. This result is discussed in the context of the associative encoding deficits characterizing aMCI. (Contains 2 figures, 2 graphs, and 1 table.)