We reviewed 294 pairs of twins born from January, 1966 to December, 1972. In 19 pairs one or both members developed hyaline membrane disease (HMD). Of these, both twins were affected in 12 pairs, twin B alone in six pairs, and twin A alone in one pair. The group affected (19 pairs) had lower gestational age, birth weight, Apgar score, increased incidence of monozygotic (MZ) twins, and higher mortality rate than the group without HMD (275 pairs). MZ twins were more immature than dizygotic (DZ) twins (p less than 0.02). When both twins were affected they had lower gestational age, birth weight, and increased monozygosity than when B alone was affected (p less than 0.05). When twin B alone was affected, he had lower Apgar score than twin A (p less than 0.05). We suggested that (1) HMD occurs in twins because of lung immaturity, as it does in singletons; (2) monozygosity may be a predisposing factor to HMD because of the associated prematurity; and (3) the greater risk of twin B is probably related to birth asphyxia.