In a long-term prospective study 46 unselected infants born before 35 completed weeks of gestational age were followed up, and compared to 26 fullterm infants. At 9 and 18 months of chronological age their height and weight were still lower than that of fullterms, but the difference disappeared when age was corrected for gestational age at birth. The motor and neurological maturity and language development was delayed in the preterms still at 18 months, which could possibly also be explained by their lower biological age. Ten of the preterm infants showed, at one or several occasions during follow up, definite neurological abnormality. At 18 months of age two of them were handicapped, one with retrolental fibroplasia, nearly blind, and another with cerebral palsy (slight spastic diplegia). Five of them had late psychomotor development, while two were borderline and one normal. We defined pre- and perinatal risk groups, but found that development at 18 months was not correlated to degree of risk. Neither was there any correlation between neurological examination at term and later handicap or psychomotor retardation. We found more illness, mostly due to common infections, during the first 18 months in the preterm group, as measured by the number of visits to a doctor and days spent in hospital.