An outbreak of pharyngoconjunctival fever caused by adenovirus (Ad) occurred in 1976 in Belem (northern Brazil). By restriction endonuclease analysis, it was shown that the type responsible was Ad4a, a variant of Ad4 prototype (Ad4p) and previously described as causative agent of ocular diseases. In an epidemiologic study carried out in Rio de Janeiro and Belem between 1977 and 1986, 148 Ads were isolated from children with respiratory disease. All five Ad4 strains encountered during this study were also Ad4a. This type, circulating in South America, can therefore cause both ocular and respiratory diseases. No Ad4p was isolated during this study. A percent of comigrating restriction fragments of only 50% between Ad4p and Ad4a was calculated from 13 restriction enzyme patterns. The restriction maps of Ad4a for nine endonucleases are presented and compared with those of Ad4p.