Twin pregnancy is considered to be a risk factor for congenital dysplasia of the hip. From 1987 until 1996, the hips of 4476 (2260 male, 2216 female) newborn babies were examined by ultrasound according to Graf's technique and classification in our hospital. In this study, we compare the results of twins and singletons for this risk factor. Of the newborns, 97 (2.2%) were twins (40 male, 57 female); 39 pairs of twins (10 male/male, 19 female/female, 10 male/female) and 19 individual twins (6 male, 13 female) were investigated. Hips of type Ia, Ib and IIa (alpha > or = 55 degrees) are not pathologic; hips of type IIa (alpha <55 degrees ) need an early control examination; and hips of type IIc, D, IIIa, IIIb and IV require therapy. Types Ia, Ib, and IIa (alpha > or =55 degrees ) were found in 4207 (94.0%) of all newborns, in 4112 (93.9%) of the singletons, and in 95 (97.9%) of the twins. Early control examination and/or therapy (indicated for types IIa (alpha <55 degrees ), IIc, D, IIIa, IIIb, and IV) were necessary in 269 (6.0%) of all cases, in 267 (6.1%) of singletons and two (2.1%) of twins. Twins with additional factors such as breech position birth, hip dysplasia in the family or premature birth did not show the types of hip IIa (alpha <55 degrees ), IIc, D, IIIa, IIIb, IV. We did find these hips in two (3.5%) of the female twins, but not at all in the male twins. Statistically, twins with or without other risk factors that are known before birth did not show significantly more of type hip IIa (alpha <55 degrees ), IIc, D, IIIa, IIIb, IV (P>0.05).