OBJECTIVE: To describe the methods used at birth to recruit a population-based cohort of newborns of all birthweights at higher risk of having a chronic condition, and to present baseline results. METHODS: Screening of all newborns at hospital discharge for five non-exclusive criteria: (1) low birthweight (LBW), (2) congenital anomalies or genetic disease, (3) specified conditions associated with a high probability of chronicity, (4) referral to a neonatal intensive care unit (NICU), (5) or defined social problems. Calculation of Hobel risk scores for children satisfying > or = 1 criterion. SUBJECTS: All 6477 live births delivered in the 19 maternity hospitals of a geographically defined region (Vaud, Switzerland) to resident mothers in 1993-1994. RESULTS: Twelve per cent (n = 760) of newborns met > or = 1 criterion: 6.3% of all newborns had an LBW (criterion 1), 2.4% had a birth defect, 0.9% met criterion (3), 4.4% stayed in an NICU and 1.6% had serious social problems. Hobel prenatal score was high (> or = 10 points) for 41% of children with > or = 1 criterion, the intrapartum score for 87% and the neonatal score for 68%. CONCLUSIONS: Most newborns identified by the above simple criteria also had elevated perinatal risks. The validity of the criteria will later be tested against the results of the examinations of children with > or = 1 criterion at 18 months and 4 years of age, but the assessment at birth already shows that normal birthweight (NBW) children, in agreement with previous studies, contribute half the children at high risk perinatally.