Risk factors were studied in 96 children with congenital hydrocephalus (CH) coming from 118,265 consecutive births of known outcome. Hydrocephalus with neural tube defects, intracranial tumors or secondary to brain atrophy were excluded. The prevalence of CH was 0.81 per thousand. Diagnosis was performed prenatally in 41 cases. Forty-three (44.8%) of the cases had hydrocephalus without other malformations (isolated hydrocephalus), 18 (18.7%) infants had recognized chromosomal or non-chromosomal syndromes and 35 children (36.4%) had multiple malformations. Each case was matched to a control. Weight and length at birth of children with hydrocephalus were less than in the controls (p < 0.001). The weight of the placenta was lower than in the controls (p < 0.05). The pregnancy with a hydrocephalic child was more often complicated by threatened abortion, polyhydramnios and oligohydramnios. The mothers of children with hydrocephalus and multiple malformations had used oral contraceptives during the first trimester of pregnancy more often than the mothers of the controls. No differences appeared between the mothers of children with CH and the controls for the other risk factors studied: parental age, parity, previous pregnancies, previous stillbirths, smoking, diabetes, epilepsy, X-rays, hypertension, fever "flu", medication and occupational exposure. There was an increase of parental consanguinity in the parents of our patients (6.2% v. 1.1%, p < 0.001) and first degree relatives had more non-cerebral malformations than the controls (7.3% v. 3.2%, p < 0.05).