Purpose Between 14% and 85% of patients report noticeable changes in cognitive function during chemotherapy (CTX). The purposes of this study were to determine which demographic, clinical, and symptom characteristics were associated with inter-individual variability in initial levels of attentional function as well as with changes in the trajectories of attentional function in a sample of oncology patients who received two cycles of CTX. Methods Oncology outpatients (n=1,329) were recruited from two Comprehensive Cancer Centers, one Veteran’s Affairs hospital, and four community-based oncology programs. The Attentional Function Index (AFI) was used to assess perceived effectiveness in completing daily tasks that required working memory and attention. Hierarchical linear modeling (HLM) was used to evaluate for inter-individual variability in initial levels and in the trajectories of attentional function. Results: D emographic, clinical, and symptom characteristics associated with inter-individual differences of attentional function at enrollment (i.e., intercept) were: employment status, functional status, trait anxiety, depressive symptoms, sleep disturbance, evening fatigue, and morning energy. Gender was the only characteristic associated with inter-individual differences in the trajectories of attentional function. Morning fatigue was the only characteristic associated with both initial levels as well as the trajectories of attentional function. Conclusions Prior to their next dose of CTX, patients reported moderate levels of attentional function that persisted over two cycles of CTX. Many of the clinical and symptom characteristics associated with decrements in attentional function are amenable to interventions. Clinicians need to assess patients for changes in attentional function and associated characteristics and recommend evidence-based interventions.