Adenovirus type 7 (Ad 7) is the serotype among the 36 recognized adenovirus types which most frequently has been associated with severe illness. Three different epidemic patterns of Ad 7 infection can be distinguished: 1) the first appears during the winter among infants with median age below two years, has characteristic symptoms of high fever and pneumonia and an outcome that may be fatal: 2) the second appears in the fall among children with median age seven years, has characteristic symptoms of high fever, pneumonia, abdominal symptoms and meningism and an outcome that is favorable; 3) and the third has been seen as acute respiratory disease among military recruits. In the United States, the last mentioned outbreaks require prophylaxis in the form of a live enteric-coated vaccine. DNA restriction site mapping demonstrated the occurrence of three distinct viral entities of Ad 7, which have been designated Ad 7 prototype, Ad 7a (the vaccine strain) and the Ad 7b genome type. In the present study, 36 isolates obtained from outbreaks with the first and the second epidemic patterns were analyzed by restriction endonucleases Bam HI and Sma I. All were identified as the newly recognized Ad 7b genome type. It is concluded that this genome type is responsible for a large portion of the severe infections caused by Ad 7. The epidemic nature of Ad 7 and the severe illness noted among infants indicate that vaccination of institutionalized infants could be considered during years when Ad 7 epidemics appear.