Of 259 patients admitted to an intensive care unit with severe acute community-acquired pneumonia, 173 had primary infections and 86 had secondary infections. The commonest organism isolated in each group was Streptococcus pneumoniae (51.3 and 36.6% of known isolates in each group respectively). Klebsiella pneumoniae was the next most common isolate (31.9 and 29.3% respectively). A variety of other Gram-negative organisms and Staphylococcus aureus accounted for most of the remaining pathogens. Based on retrospective analysis of data, there appeared to be no difference in the alcohol consumption of patients with infection due to S. pneumoniae and K. pneumoniae. The overall mortality rate for the primary infections was 47.4%, with 68.4% of these infections due to K. pneumoniae and 33.9% due to the pneumococcus (P < 0.002). Among the secondary infections, the overall mortality rate was 40.8% (not significantly different to that of primary infections) with 45.5% due to K. pneumoniae and 23.1% due to the pneumococcus (not significantly different on statistical analysis, probably due to low patient numbers). Our investigation confirms that severe community-acquired pneumonia due to K. pneumoniae is extremely common, even in patients without obvious risk factors for Gram-negative colonization. This organism is contributing to the high mortality rate seen in our intensive care unit among patients with pneumonia, and our empiric therapy for such cases routinely includes a combination of agents active against this organism (e.g. a cephalosporin and an aminoglycoside).