Early diagnosis of Legionnaires' disease is difficult because other pathogens cause a similar clinical picture and microbiologic tests are usually only of retrospective value. Since May 1977, 17 patients with sporadic cases of Legionnaires' disease have been admitted, all previously well, the diagnosis being made with standard serologic or bacteriologic criteria. From the clinical, laboratory, and radiologic findings, we propose criteria that may enable the clinician to make a diagnosis earlier in many cases, differentiating them from other pneumonias. Within 24 hours of admission, any three of the following four features are strongly suggestive of Legionnaires' disease: [1] prodromal "viral" illness, [2] dry cough or confusion or diarrhoea, [3] lymphopenia without marked neutrophilia, [4] hyponatremia. Two thirds of cases had at least three of these features, and no false-positive diagnoses would have been made in other pneumonias that were serologically negative for Legionnaires' disease if these proposed criteria had been applied diagnostically. In the next few days the diagnosis is very likely if microbiologic tests are negative and if there is radiologic extension, abnormal liver function test results, or hypoalbuminemia.