Meningococcemia caused by serogroup W135 occurred in four patients during a period of nine months. Three of the patients had hypogammaglobulinemia with normal hemolytic complement levels; the fourth had normal immunoglobulin levels one year after her illness. None of the patients had classic meningitis. One patient had pneumonia; one appeared to have encephalitis with mild meningismus; and two had sepsis without localizing signs or symptoms. Patients with hypogammaglobulinemia are predisposed to the development of meningococcemia, and the incidence of W135 carriage in the population is increasing.