The authors studied nursing home residents serologically to determine whether atypical organisms were causes of radiologic pneumonia. The study was conducted at the Wisconsin Veterans Home, a facility with on-site microbiology and x-ray. Over one year, serologic examinations for Legionella, Mycoplasma, and Chlamydia were conducted for the residents who had pneumonia. Cultures and mortality were reviewed. Fifty-six episodes were studied (mean resident age 78 years). There was no fourfold titer change. Seventeen quality sputum specimens revealed Streptococcus pneumoniae (5), normal flora (4), Hemophilus influenzae (4), Moraxella catarrhalis (3), Staphylococcus aureus (1), and beta-hemolytic Streptococcus, not group A (1). The two-month mortality was 21%. This study did not result in serologic confirmation of atypical organisms' causing pneumonia. Antibiotic choice should be based on coverage of prevalent organisms, including Hemophilus influenzae, Moraxella, and Staphylococcus, as well as clinical features.