The clubfoot deformity has been known for approximately 3000 years, but the causes of clubfoot remain unknown. This study will examine a recently developed hypothesis that exogenous and endogenous mechanical factors may act together in the causation of clubfoot. If mechanical factors are shown to be related to occurrence of clubfoot, a rationale for primary prevention of this common and crippling deformity may be developed. This population-based case-control study will investigate the association between ligamentous laxity, intrauterine constraint, family history, and birth prevalence of clubfoot in Washington State. Cases will be ascertained through the Washington State Birth Defects Registry, which was established in 1986. Registered children will be included if they were born and diagnosed as having isolated talipes equinovarus (TEV) or isolated talipes calcaneovalgus (TCV) between October 1, 1990 and March 31, 1994. For each case, a control will be randomly selected from birth certificates of infants born to state residents during the same month. The mothers of case and control infants will be interviewed for detailed information on indicators of intrauterine constraint, ligamentous laxity in the child and relatives, family history of birth defects, and potential confounding factors. Particular attention will be paid to identification of exposures which may be modified by preventive intervention. In addition, mothers will be asked to briefly examine family members for mild double- jointedness, and a study nurse will examine family members residing in the five-county area around Seattle. Medical records will be reviewed for information on factors related to intrauterine constraint. Since TEV and TCV are likely to be multifactorial in etiology, and to have different risk factors, the relations between these deformities and ligamentous laxity, family history of clubfoot, and intrauterine constraint will be evaluated using polychotomous logistic regression. The possibility that ligamentous laxity and family history modify the effects of exogenous mechanical factors will be evaluated, as well the possibility of confounding by other factors. This population-based case-control study will provide the most efficient and practical method for studying the mechanical hypothesis in humans. Studies in humans are especially important to carry our because of the species-specific nature of mechanical factors, especially those related to physical activity.