To test a method of studying incidence of preschool morbidity, based on absences from school among 34 three-, four-, and five-year-olds at a child study center on a university campus from January to 15 May 29, 1968, the parent was contacted and a questionnaire was administered on the first day of the child's absence and on every third school day thereafter until the child recovered. A household informant of the 23 families of the children also was questioned to study clustering of illness surrounding, by a two-week period, the child's first day of absence during the month of March. Of the 153 absences reported, 87 resulted from illness. One epidemic of a communicable diseas, mumps, occured during the study period. Findings are reported for number and percentage of symptoms as perceived by the parent; types and distribution of illnesses in the children; person-days at risk, incidence, and duration of illnesses by age, sex, school year, family groups, and child's physician; incidence and duration of illnessed by month; distribution and duration of illness according to type of care; standard incidence ratios for family groups; and incidence and distribution of illness by family functioning. The questionnaire used in the study was found to be practical and valid, but the method of studying clustering of disabling illness within families was not recommended.