The objective of this study is to investigate if changes in cognitive functions can be recognised in patients undergoing chemotherapy for breast cancer. Forty women with breast cancer and without depression underwent cognitive evaluation before and after 6 months of chemotherapy; emotional evaluation was performed before and after 1, 3 and 6 months of chemotherapy. Self‐reported cognitive deficit evaluation was included. Global cognitive functioning before starting chemotherapy was good. After 6 months of treatment there was a significant decline in some cognitive functions, particularly involving the attention subdomain. Objective cognitive deficit resulted independent from the emotional status. On the contrary, self‐perceived mental dysfunction was unrelated to the objective cognitive decline, but it was associated with depression and anxiety. Breast cancer chemotherapy can induce domain‐specific cognitive dysfunction. Patients' self‐perception of mental decline is unrelated to objective cognitive deficit. Breast cancer patients negatively judge their cognitive performances if they have a negative emotional functioning.