A recently recognized sporadic case of "legionnaires' disease" occuring in Minnesota is described. The patient, a previously healthy 58-year-old woman, became seriously ill with progressive pneumonia in the spring of 1977. Her illness was unresponsive to the usual antibiotics (penicillin and cephalothin) prescribed for community-acquired bacterial pneumonia. Treatment with doxycycline appeared to be effective. Diagnosis was made by special serologic studies performed at the Center for Disease Control, Atlanta. As reported in the epidemic form of the disease, no secondary cases were noted in family, associates, or hospital workers exposed to the patient during her acute illness. This disease may be more common than has previously been recognized, for serologic test for the unusual bacterium have only recently become available.