Risk factors were studied in 118 severely small for gestational age (SGA) infants. The control infants were matched for gestational age and mode of delivery. A low maternal prepregnancy weight and a maternal history of a previous SGA infant were the most important prepregnancy risk factors, whereas poor weight gain during pregnancy, toxemia, and smoking were the most important pregnancy-related risk factors associated with severely SGA infants. The two groups were similar for primiparity, previous abortions, and placental abnormalities in mothers and for parental ages and heights. Thirty percent of SGA infants were diagnosed antenatally. Most of the risk factors cannot be avoided, but the effects of these risk factors on the fetus and neonate can be minimized by careful screening and observation of mothers with these risk factors.