A retrospective study on 834 newborns of low birth weight, 2,500 grs., or less over a total of 19,869 live newborns during the year 1973 is presented. They are divided into low birth weight compatible with gestational age and low birth weight for gestational age when birth weight is lower than 10th percentile of Lubchenco's chart. Each of these groups is subdivided in three parts: First part refers to coincidence of frequence of different ethiologic factors or causes of low birth weight (maternal, gestational, placental and termination of pregnancy). The second part has its object in the clinical appraisal of the state at birth of these low birth weight newborns (fetal-obstetric evaluation of Velasco-Cándano, neonatal Apgar scoring, respiratory evaluation by Silvermann-Andersen and evaluation of external maturaty according to Usher). Third part of the study relates the morbility-mortality rate, need for admission, presence of congenital anomalies and autopsy findings in each of these groups of low birth weight newborns. Relation among all these estimated parameters is made presenting some conclusions reached.