Birth weight of 1,134 twin pairs have been studied in the Maternity "La Paz" (Madrid) along years 1969-74. Mean weight among twin pairs of the like sex is 2,493.78 +/- 20.89 for males and 2,425.16 +/- 19.16 for females. Among the unlike sexes the means were 2,631.07 +/- 28.38 for males and 2,488.16 +/- 26.05 for females. Differences among the means were all statistically significative. This results have been compared with those obtained in other populations. Variability of mean weights acording with gestational age has been studied also (from 28 to 43 weeks). Curves obtained in this way were supperposed to those of single birth; until the 37 week, twins are comprised between the means and on standard deviation corresponding to single birth distributions. Increment weight per week among twins is higher than those corbresponding to single births until the 34 week. From these week growth becames slow. From the 39 week twins lost weight in a fast way. Weights differences among twin pairs grow with gestational age, being very significant from the 36 week.