The effects of gender on mortality and morbidity of all neonates weighing less than 1500 g admitted to King's College Hospital Neonatal Intensive Care Unit during 1980-82 (n = 271) were examined. Very low birthweight boys had a significantly higher mortality and more postnatal complications than girls. The higher incidence of respiratory distress syndrome and pulmonary interstitial emphysema in boys was associated with increased mortality in the first year. Surviving boys had significantly more problems, including lower Apgar scores at five minutes, more frequent apnoeic attacks and bradycardic episodes, transient tachypnoea, neonatal anaemia, and lower blood calcium and phosphate concentrations. Surviving children were followed up at 1 and 2 years of age. Development of boys at 1 year was significantly delayed compared with girls in all fields save locomotor. Although at 2 years some of the differences had diminished, those in language and personal social skills were more pronounced. More than twice as many boys as girls had major neurodevelopmental disorders.