ImportanceReturn to work after breast cancer (BC) treatment depends on several factors, including treatment-related adverse effects. While cancer-related cognitive impairment is frequently reported by patients with BC, to date, no longitudinal studies have assessed its association with return to work.ObjectiveTo examine whether cognition, assessed using objective and subjective scores, was associated with return to work 2 years after BC diagnosis.Design, Setting, and ParticipantsIn a case series of the French Cancer Toxicities (CANTO) cohort, a study of patients with stage I to III BC investigated cognition from April 2014 to December 2018 (2 years’ follow-up). Participants included women aged 58 years or younger at BC diagnosis who were employed or looking for a job.Main Outcomes and MeasuresThe outcome was return to work assessed 2 years after BC diagnosis. Objective cognitive functioning (tests), cognitive symptoms, anxiety, depression, and fatigue were prospectively assessed at diagnosis (baseline), 1 year after treatment completion, and 2 years after diagnosis. Multivariable logistic regression models were used to explain return to work status at year 2 according to each cognitive measure separately, adjusted for age, occupational class, stage at diagnosis, and chemotherapy.ResultsThe final sample included 178 women with BC (median age: 48.7 [range, 28-58] years), including 37 (20.8%) who did not return to work at year 2. Patients who returned to work had a higher (ie, professional) occupational class and were less likely to have had a mastectomy (24.1% vs 54.1%; P < .001). Return to work at year 2 was associated with lower overall cognitive impairment (1-point unit of increased odds ratio [1-pt OR], 0.32; 95% CI, 0.13-0.79; P = .01), higher working memory (1-pt OR, 2.06; 95% CI, 1.23-3.59; P = .008), higher processing speed (1-pt OR, 1.97; 95% CI, 1.20-3.36; P = .01) and higher attention performance (1-pt OR, 1.63; 95% CI, 1.04-2.64; P = .04), higher perceived cognitive abilities (1-pt OR, 1.12; 95% CI, 1.03-1.21; P = .007), and lower depression (1-pt OR, 0.83; 95% CI, 0.74-0.93; P = .001) at year 2 assessment. Return to work at year 2 was associated with several measures assessed at baseline and year 1: higher processing speed (1-pt OR, 2.38; 95% CI, 1.37-4.31; P = .003 and 1.95; 95% CI, 1.14-3.50; P = .02), higher executive performance (1-pt OR, 2.61; 95% CI, 1.28-5.75; P = .01, and 2.88; 95% CI, 1.36-6.28; P = .006), and lower physical fatigue (10-pt OR, 0.81; 95% CI, 0.69-0.95; P = .009 and 0.84; 95% CI, 0.71-0.98; P = .02).Conclusions and RelevanceIn this case series study of patients with BC, return to work 2 years after diagnosis was associated with higher cognitive speed performance before and after BC treatment. Cognitive difficulties should be assessed before return to work to propose suitable management.