The possible role of infection with Mycoplasma pneumoniae (MP) in exacerbation of bronchial asthma in adults was studied in 95 patients hospitalized due to acute asthma. Twenty (21%) of these patients had evidence of a recent MP infection as determined by the presence of high levels of MP-specific IgM antibodies. In addition, high levels of both IgM and IgG but not IgA serum immunoglobulins were observed in the MP-infected group as compared to a control group of 20 non-MP-infected asthmatics. Five out of 20 MP-infected asthmatics exhibited rheumatoid factor (RF) in their sera while patients in the control group were all negative for RF. It is concluded that MP infection may be significant in exacerbation of asthma in adults.