In a prospective survey, 420 out of 502 infants of low birth weight (≶ 2,041 gm, or 4½ lb) and all but one of 207 control children of full birth weight (> 2,500 gm, or 5½ lb) have survived for more than 3 years. One hundred fifty of the low birth weight (LBW) group have passed the age of 6 years. Developmental and psychological tests have given the following results: (1) Control children performed better than LBW infants on the Griffiths Developmental Scale up to the age of 18 months; among the LBW infants mean Griffiths Scores in 250 gm birth weight groups mosty differed significantly in direct relation to the weight. (2) LBW girls scored higher than boys after the first year on the hearing-speech subscale and, to a lesser extent, on the personal-social and performance subscales of the Griffiths test and also on the Stanford-Binet and Graham-Ernhart tests at 4 years of age. (3) Whereas the effect of birth weight on I.Q. became less distinct at 2½ to 4 years, the effect of socioeconomic status only became definite at that age. (4) In general, "small-for-dates" (SFD) children, including those born before term, scored higher than "true prematures" up to the age of 12 months and slightly lower at 2½ to 6½ years, but the differences were only significant in a few weight groups. When the SFD children were subdivided into those born at less than 37 weeks' gestation and those born later, the latter scored significantly better than the former only at 3 and 6 months. (5) Isolated Griffiths infant test scores at 6 months per se have little predictive value for I.Q. scores of children at 4 years of age, even at the extremes of intelligence.