A group of 34 twin pairs was studied at four years of age. Characteristics of the group were a homogeneous social background, good parental interrelationship and conscientious care during pregnancy, delivery and the neonatal period. All the children were tested with the Griffiths' Mental Development Scales. The girls generally showed a higher development quotient than the boys. Birth weight seemed to be the most important factor; at 4 years of age, the twins with a birth weight of less than 2500 grams had a significantly lower locomotor quotient. The language quotient showed a similar trend. This difference was, however, not significant. No significant differences in either locomotor or language development could be observed between MZ and DZ twins.