PIP: Researchers analyzed data from the National Child Development Study--a cohort of every child born in England, Scotland, and Wales during the 1st week of March 1953 with follow up studies in 1965, 1969, 1973, and 1980-1981 to examine the relationship between health status and birth order and whether children with low birth orders were less likely to experience illness than those with older siblings. 1st born children tended to have received the needed number of immunizations, but children of higher birth order did not tend to have received them. Further they were more likely to have attended infant welfare and toddler clinics for health care than children of higher birth order. The only childhood contagious disease which demonstrated a social class effect was pertussis. It tended to afflict children from nonmanual homes regardless of birth order. Absences from school lasting between 1 week-1 month of 1st born children were less frequent than for other children. The leading reasons for 1st, 3rd, and later born 11 year old children who experienced such long absences included infectious diseases; bronchitis; ear, nose, and throat complaints; pneumonia; tonsillitis, or viral influenza. After age 15, 1st and 2nd born children were less likely to be absent and, if absent, they tended to only miss 1 week of school. Significantly more 3rd and 4th born children were absent from school for 1 week-3 months. 1st and 2nd born children from more affluent families tended to have early childhood asthma. In conclusion, the health experiences of the later birth orders were different than those of the 1st born. This did not mean, however, that later birth order children were in poorer health than 1st born children.