Fox, J. P. (Univ. of Washington School of Public Health and Community Medicine, Seattle, Wash. 98195), C. E. Hall, M. K. Cooney, R. E. Luce and R. A. Kronmal. The Seattle Virus Watch. II. Objectives, study population and its observation, data processing and summary of illnesses. Am J Epidemiol 96: 270–285, 1972.—From Nov. 1965 to Aug. 31, 1969 the Seattle Virus Watch (VW) concentrated on the continuing surveillance of families with newborn infants for infections with respiratory or enteric viruses recoverable in cell cultures, chiefly human embryonic kidney and WI-38 human diploid. Two groups of families were recruited for 2-year periods from the clientele of a large prepaid comprehensive medical care plan. Group 1: in scheduled biweekly visits, respiratory and fecal virus isolation specimens were collected from infant, mother and next older sibling, if any, plus the interval illness record kept by the mother. Special visits were made and similar specimens collected from the patient and other available members when illness occured in any family member. Group 2: “telephone families” provided weekly information by phone and, no more than twice a year, were studied virologically when a new illness suggested an impending family episode. The aim was to expand the basis for describing the spectrum of illnesses associated with specific viral infections; however, observations of telephone families ended Sept. 1968 due to the small number of virus-associated illnesses recognized. Both groups gave venipuncture bloods for serology on admission and every 6-months thereafter except for infants who were first bled at 18 months. The 149 families in group 1 (85% completed the study period) contributed 11,583 person-months of observation, 32,604 virus isolationand 2211 serum specimens. The 66 telephone families in group 2 (51% withdrew before 2 years) contributed 5177 person-months of observation, 767 virus isolation and 669 serum specimens. Reported illnesses in telephone families were much less frequent than in the first group (2.3 vs. 4.1 illnesses per person-year). This difference was unrelated to age and the distributions of illnesses by classification were similar. The Seattle and New York VW studies were about equal in duration, person-months of observation and specimens accumulated. However, the New York study population was less stable (64 of 175 families withdrew); the median observation period was 11 months compared with 20 in Seattle and coverage of illness by virus isolation specimens was less complete (62% in New York, 87% in Seattle for respiratory illnesses). Illness patterns also were slightly different. Despite underreporting of paternal illnesses in New York, the overall illness rate exceeded that in Seattle (4.8 vs. 4.1 per person-year). Respiratory illnesses were greater in Seattle (86 vs. 78%) whereas a higher proportion of illnesses were enteric in New York (15 vs. 8%). The Seattle VW benefited from more sophisticated computer assistance.