Molecular targeted therapy is medical treatment targeting specific molecules, which are essential in the pathology of diseases. Most agents used are monoclonal antibodies ("biologics") and low-molecular-weight compounds. Molecular targeted therapy is widely utilized against malignancies, such as imatinib for chronic myelogenous leukemia blocking BCR-ABL tyrosine kinase, gefitinib for non-small cell lung cancer interrupt- ing signal transduction through EGFR, and trastuzumab for HER2-positive breast cancer. It is a companion diagnostic used as a companion to a molecular targeted drug to determine its applicability for a specific patient, showing the importance of laboratory tests for cancer treatment. While the pathogeneses of connective tissue diseases are still unknown, recent progress in understanding the pathophysiology enables us to use molecular targeted drugs for effective treatment. Rheumatoid arthri- tis (RA) is the most common connective tissue disease, and inflammatory cytokines such as TNFa and IL-6 play a pivotal role in the pathogenesis of RA. In Japan, successful treatment of RA with a chimeric antibody to TNFa(infliximab) is followed by a number of anti-cytokine drugs, such as humanized anti-TNFa, anti- TNF receptor, and anti-IL-6 receptor antibodies. A fusion protein (abatacept), an inhibitor of activated T- cells, composed of the extracellular domain of CTLA-4 and the Fc region of IgG1, and more recently Janus kinase inhibitor (tofacitinib), have also been demonstrated to be highly effective. Since molecular targeted therapy suppresses the immune function, patients receiving the therapy become susceptible to infection. Thus, clinical laboratory tests are of great importance, not only to classify potentially high-risk patients (especially in the case of the so-called post-infectious state of tuberculosis and hepatitis B to avoid reactivation) before treatment but also for the early detection of infections during treatment. [Review].