Thirteen cases of Caplan's syndrome were investigated by chest X-ray, rheumatic and immunological tests, heart and lung physiology and pathological-anatomical specimens. No positive correlation was found between exposure time to silica, roentgenolgical findings, degree of rheumatoid factor or physiological findings. In nine of the cases a rapid roentgenological progress took place. Although the combination of lung infiltrations and rheumatic disorders makes an immunological reaction probable, no correlation was found between the degree of rheumatic disorder and roentgenological progress. The hypothesis that silica acts as an adjuvant, which may strengthen an antigen-antibody reaction, was not reflected in rheumatic parameters. The roentgenological examination appeared to be the most sensitive method for following the disease.