OBJECTIVE To measure IgG antibody subclasses in previously healthy adult patients with acute community-acquired pneumonia, and to assess any association between differences of subtype levels and severity of illness or prognosis. METHODS Prospective study. METHODS The intensive care unit (ICU) and general medical wards of Hillbrow Hospital, Johannesburg, an urban general hospital. METHODS Sixty-six previously healthy adult patients with acute community-acquired pneumonia, of whom 47 were considered less severely ill, while 19 were admitted to an ICU. METHODS Measurement of IgG subclass levels and determination of any association between differences in subtype levels and various poor prognostic factors in pneumonia, need for ICU admission, complications of illness, and APACHE II score of ICU cases or outcome of patients. RESULTS A number of statistically significant differences (P < 0.05) were noted between the two groups of patients (critically ill v. others) representing well-known negative prognostic factors in pneumonia. A greater degree of tachycardia and tachypnoea and extremes of white cell count, a higher serum urea concentration and multilobar pulmonary consolidation characterised the patients in the ICU. In addition, the mortality rate in the ICU patients was significantly greater (P < 0.0001). Similar findings were noted when survivors and non-survivors were compared. Few abnormalities of IgG subclass levels were noted in the various patient groups, which did not allow adequate analysis of their clinical significance. CONCLUSIONS This study demonstrated a small number of abnormalities in IgG subclass levels in previously healthy adult patients with acute community-acquired pneumonia.