This article reports the results from an ERP study on the processing of anaphoric reference to quantifying expressions in Swedish (e.g. Many students attended the lecture and that they were present was noted). Negative quantifiers (e.g. few) differ from positive quantifiers (e.g. many), in allowing anaphoric expressions to target either the ref(erence) set (‘students attending the lecture’) or the comp(lement) set (‘students not attending the lecture’), while positive quantifiers only allow Refset continuations. Results from the present study show that negative quantifiers give rise to an enhanced frontal negativity at the anaphoric pronoun in the negative condition, relative to positive quantifiers. At the critical word disambiguating between a Refset and Compset reading, we found P600 effects for the anomalous relative to the non-anomalous conditions. We interpret the frontal negativity found with negative quantifiers as an indication of referential ambiguity interfering in the processing of anaphoric reference.