We investigated four patients in whom histologically confirmed Burkitt's lymphomas developed during a one-year period. They were young adults living within 50 km of each other in a rural area of Pennsylvania. Two patients were related by marriage, but there was no contact with or between the other two. Although the majority of American Burkitt's lymphomas are not associated with the Epstein-Barr virus, all four patients showed spectra and titers of antibodies to antigens related to that virus that were characteristic of virus-associated Burkitt's lymphoma in Africa. Viral DNA and numerous cells with virus-determined nuclear antigen were found in tumor specimens available from three patients. These cases are unusual, since time-space clustering of Burkitt's lymphoma associated with Epstein-Barr virus has been observed thus far only in Africa.