The diagnosis and treatment of pneumonia in mass gathering situations is a medical challenge, requiring prompt decision making and knowledge of the aetiology. We studied cases of pneumonia admitted to two hospitals during the 1994 pilgrimage (Hajj) season to Makkah. Sixty-four patients were enrolled in the study, of which 47 (75%) were men with a mean age of 63 years (range 21-91). Nearly all were from developing countries. Diagnosis was established in 46 patients (72%) with Mycobacterium tuberculosis being the commonest causative organism (20%), followed by gram-negative bacilli (18.8%). Streptococcus pneumoniae accounted for only 10%, with Legionella pneumophilia, Mycoplasma pneumoniae, and viruses accounting each for 6%. The main finding of this study is that M. tuberculosis is a common cause of pneumonia under these unusual "extreme circumstances". Its presentation was acute and indistinguishable from pyogenic pneumonia. Thirty-one per cent of tuberculous cases had upper lobe involvement, 54% lower lobe, and 15% multi-lobar. This was similar to the radiographic features in non-tuberculous pneumonia cases. All but one patient with tuberculosis recovered following the administration of first-line anti-tuberculous drugs. The total mortality was 17%. The preponderance of M. tuberculosis and Gram-negative bacteria over S. pneumoniae may reflect the prior use of amoxycillin and the effect of exhaustion, malnutrition, and old age.