Eighteen families were followed up for four to six weeks after one member of each family was diagnosed as having an adenovirus infection. In 17 of 18 index cases the diagnosis was based on the rapid detection of adenovirus hexon antigen in the nasopharyngeal mucus specimens and in one case (the only adult index case) on isolation of the virus. All index cases had high temperatures associated most commonly with tonsillitis, acute otitis media, gastroenteritis, or febrile convulsions. In 14 of the 16 families with symptomatic contacts the index case was the first symptomatic case, or one of the first symptomatic cases, in that family. Fifteen (94%) of the siblings and 20 (56%) of the parents had signs and symptoms of acute infection during the follow up period. In 10 (63%) and eight (20%) of these cases, respectively, adenovirus was confirmed. The mean (SD) incubation period of confirmed adenovirus infections was 10 (3) days. The observations show that adenovirus infection spreads actively to other siblings in the family. Rapid diagnosis permits parents to be informed prospectively about the expected spread and clinical picture of the illness in the family.