This study examined the association of the children's entry age and enrollment duration of Head Start on children's mental health, using secondary data analysis. Children were of three groups: one group of children entered Head Start at age 3 and maintained enrollment for 1 year (Group 1), the second group of children entered Head Start at age 4 and stayed for 1 year (Group 2) and the third group of children entered at age 3 and stayed in Head Start for 2 years (Group 3). Research questions are (1) Do child and family characteristics have any association with mental health scores? (2) Do children's mental health scores differ among the three groups? Compared with children who entered at age 4 and stayed for 1 year, children who entered Head Start at age 3 and stayed in Head Start both for 1 year and for 2 years had higher mental health scores. Controlling for other factors, boys and children with special needs had higher mental health scores. Black and dual bilingual children had lower mental health scores. (author abstract)