The Edinburgh Register of the Newborn 1964-1968 and the Edinburgh Scoliosis Clinic 1964-1971 have been used to establish the population frequency in the city of the idiopathic forms of talipes equinovarus and calcaneovalgus, metatarsus varus, congenital dislocation of the hip, and infantile scoliosis. A survey of 165 patients now aged 7 to 11 years showed an aetiological relationship, but with differing environmental factors. These factors were established by comparison with the Edinburgh Register control group of 692 normal infants born over the same period, giving a unique opportunity to obtain more accurate antenatal data than has previously been possible. The principal associations were: talipes equinovarus with antepartum haemorrhage and maternal hypertension; metatarsus varus with twin pregnancies; congenital dislocation of the hip with first born children, older than average fathers, breech presentation, a significant lack of menstrual problems in the mother, and maternal upper respiratory infection during pregnancy; infantile idiopathic scoliosis with breech presentation, prematurity, and the onset of the curve in the winter months. No significant association with raised intrauterine pressure (hydramnios or oligohydramnios) was found among these simple idiopathic deformities. It is concluded that the multifactorial genetic background in likely to be similar in all, but that the additional environmental element is variable.