Opportunities and difficulties in long-term studies of growth.

Overviews are presented of the University of Melbourne Child Growth Study and of the Fels Longitudinal Study; each of these studies had a major focus on the growth and maturation of children. Planning, study design, logistics and staffing varied between these studies, but each led to exciting research opportunities that can exist only when longitudinal data are available. There are important differences between these studies in the types of participants, ages at examinations, the variables measured, some topics of central concern, and in funding. In each study, research productivity was low in the early years, the retention of participants was a major concern, and it was necessary to maintain a balance between data collection, data management, and data analysis. More such studies are needed that focus on particular population groups and topic areas and that utilize the lessons of the past to maximize the chances of future success.