We identified 410 infants who were not walking independently by the age of 18 months from among a geographically defined population of 4275 infants who either were born weighing less than 2000 g or had needed admission to the special care nursery in the neonatal period. The outcome of the late walkers was ascertained at the age of 3 years by health visitors using a standard questionnaire. Of the late walkers, 230 (56%) had an associated abnormality diagnosed before the age of 3 years, and in 77 of these children (33%) this was definite or suspected cerebral palsy. The high prevalence of late walking among infants born before 28 weeks' gestation (46%) was almost entirely accounted for by a high incidence of impairment. Late walking is a simple marker of morbidity in this group of infants.