A longitudinal study of a group of Manhattan residents was performed in the Cornell Family Illness Study. Weekly observations of 14 acute symptoms were made by interview. The symptoms were combined into syndromes by occurrence, and episodes of syndromes were considered illnesses. Analysis of illness in this population showed results similar in incidence, frequency, and symptoms to those of a number of similar studies, despite differences in definitions and methods. The 1,707 subjects of the study yielded 1,168 person-years of information. Respiratory illnesses accounted for 60 per cent of all illnesses and 81 per cent of the person-days of illness. The average respiratory illness lasted 9.4 days (6 days longer than the average nonrespiratory illnesses). The incidence of “common cold” was 3.4 per person per year; the incidence of all respiratory illnesses was 4.6 per person per year. The rate decreased with age, and those younger than 5 years had twice the rate of that in older persons. The rate for women was greater than that for men, and the rate for whites was greater than that for either Negroes or Puerto Ricans.