We prospectively studied 102 patients, aged 15-50 years, with acute community-acquired lobar pneumonia without underlying cardiorespiratory illness, admitted to Baragwanath Hospital May 1990-April 1991. Demographic, clinical, microbiological and laboratory data and negative prognostic features of these patients are described. In particular, we documented electrocardiographic changes and studied their possible relevance in patients with pneumonia. Electrocardiographic changes occurred in 32 patients (31%). The commonest changes were clockwise rotation (16%), followed by P. pulmonale (9.8%) and S1 Q3 T3 pattern (7.8%) Other changes included right axis deviation (n = 6), right bundle branch block (n = 3), ventricular extrasystoles (n = 2), atrial fibrillation (n = 1) and nodal rhythm (n = 1). These changes returned to normal in survivors after a mean of 2 days. The S1 Q3 T3 pattern was associated with cardiac enzyme leak (CK-MB fraction), hypoxia and a high Simplified Acute Physiology Score (SAPS). In addition, P. pulmonale, right axis deviation and clockwise rotation correlated with hypoxia and a high SAPS score. Clockwise rotation also correlated with serum (including cardiac fraction) enzymes leak (LDH and CK-MB fraction), and pulmonary artery pressure. The overall mortality rate was 10.8%, with no association between electrocardiographic changes and mortality. The negative prognostic factors documented were hypoxia (p < 0.0001), multilobar pulmonary consolidation (p < 0.0001), tachycardia (p = 0.0001), tachypnoea (p = 0.0002), renal dysfunction function (p = 0.0009), hypotension (systolic p < 0.02, diastolic p < 0.003), bacteraemia (p = 0.003), and serum (including cardiac fraction) enzymes leak: LDH (p < 0.02), CK (p < 0.002) and CK-MB fraction (p = 0.0002). These factors, with the exception of renal dysfunction, also correlated with the need for intensive care unit admission. Acute and reversible electrocardiographic changes are common in acute community-acquired lobar pneumonia. Electrocardiographic changes, especially those compatible with acute cor pulmonale and accompanied by cardiac enzyme (CK-MB fraction) leak, correlated with severity of illness but not with mortality.