In mid-January 1970 a number of patients entered Evangel Hospital at Jos, Nigeria, with a severe, acute febrile illness, clinically similar to cases of Lassa fever as first seen at Jos in February 1969. By mid-February 1970 there were 26 suspected cases, some of them mild, with 10 deaths. The patients (17 females, nine males) were aged 5 months to 46 years; all but one were Nigerians. Additional Nigerian patients were seen at Vom Christian Hospital, 13 miles from Jos. Isolation of virus was not attempted. Eleven cases from Jos and two from Vom were provisionally confirmed as Lassa fever by complement-fixation (CF) test; one of these Vom patients died of “nephritis” after recovery from the original illness. In CF tests with Lassa virus antigen and serum samples from 53 persons who were household contacts of five patients, four samples were positive—two from persons with a history of minor illness, two with no history of illness. We infer that Lassa fever may include a spectrum of illness, ranging from no symptoms through mild disease to a very serious and often fatal illness.