Advances in chemotherapy have improved the prognosis of patients with breast cancer significantly. Individualization is important for optimization of chemotherapy. The prediction of tumor sensitivity to anticancer agents has been intensively investigated for that purpose. There have been 2 approaches to predict the efficacy of drugs against individual tumors, drug-sensitivity tests and molecular marker genes. Although some of these tests are already available clinically, the prediction of chemosensitivity remains a goal to be achieved. Several studies with microarrays revealed that comprehensive analyses of genes may provide useful information for determining the chemosensitivity of cancer. We have started to use a cDNA microarray to study the chemosensitivity of breast cancer. Taken together with recent data, studies for drug sensitivity should provide insights into the mechanisms of drug sensitivity and the optimal design of more effective treatment strategies in breast cancer.