Phase I, the Planning Phase, began in 1977. The protocols and Manual of Operations were developed in this phase. Phase II, the Enrollment Phase, began in March 1979 and ended in March 1981, except for infants younger than 6 months of age, who continued to be enrolled after March 1981. In 1983 the study was extended for an additional five years through 1988. Beginning in 1989, as a separate initiative, the study follows selected patient groups from the original study for an additional five years. The selected groups include the newborn cohort and patients surviving beyond age 35.