The objective of this study is to investigate whether rituximab (RTX), a drug commonly prescribed in patients with rheumatoid arthritis (RA) or inflammatory myositis, is associated with subclinical interstitial lung disease (ILD). Surrogate markers for interstitial lung diseases are a decrease of the forced vital capacity (FVC) and/or the diffusing capacity of the lung for carbon monoxide (DLCO). However, the measurement of these lung function parameters is indicated as regular follow-up examinations during the treatment with RTX according to the principles of "good clinical practice". We will perform pulmonary function testing (spirometry) including measurement of the DLCO in patients with RA, who are going to be treated with RTX. A baseline lung function measurement will be performed immediately before RTX therapy is started. Follow-up measurements will be performed at 2 weeks, 4 weeks, 8 weeks and 6 months after initiation of RTX therapy. A reduction in forced vital capacity (FVC) of >10% or a fall of >15% in DLCO will be defined as indicative for ILD.