Height and weight were measured on 299,303 children involved in eleven London County Council Surveys dating from 1904 to 1966. This paper describes the historical background to these surveys, the secular trend towards increasing heights and weights and a decreasing age at menarche, and the variation of height and weight between parts of the county. All the surveys were cross-sectional, but that of 1966 included yearly velocity data on 13,806 children. Means for height and weight by age and sex were adjusted to the exact half year to allow comparison of all surveys. Menarcheal age was determined by probit analysis on status quo data in 1954, 1959 and 1966. Intra-county comparisons were based on the Divisional Index of the nine Area Health Divisions of the County. The results indicate an end to the positive secular trend for height and weight at about the same time as the previously reported end to a decreasing age of menarch in London girls. Intra-county comparisons indicated different rates of secular trend within different areas favouring those children who were the smallest and lighest in 1949. The end of the secular trend is thought to be due to genetic factors and intra-county changes to improve environmental conditions.