Cognitive deficits are found in a number of breast-cancer patients who have undergone adjuvant (Cyclophosphamide, Methotrexate, and 5-Fluorouracil (CMF)) chemotherapy, but the underlying mechanisms are still unclear. The objective of this study is to investigate information processing in these patients with concurrent registration of brain activity. Twenty-six breast-cancer patients treated with adjuvant CMF chemotherapy and a control group of 23 stage I breast-cancer patients not treated with chemotherapy were examined. Mean time since treatment for the CMF patients was 5.1 years after the last CMF course, and for the control patients 3.6 years after termination of radiotherapy. An information processing task was administered with concurrent EEG registration. Reaction times and the amplitudes and latencies of an Event Related Potential component (P3) in different task conditions related to input, central, and output processing of information were studied. Significant differences in latency and amplitude of the P3 component were found between the treatment groups with an earlier and reduced P3 in the chemotherapy group. Patients treated with chemotherapy had longer reaction times (although not significantly different) than the control group on all task conditions. Our data provide further evidence for long-term neurocognitive problems in breast-cancer patients treated with adjuvant (CMF) chemotherapy and offer new information regarding abnormalities in brain functioning in these patients.