In a prospective investigation, 247 patients underwent ultrasonically guided midtrimester amniocentesis. Maternal serum alpha-fetoprotein (AFP) was measured before and after the procedure. In 229 patients, preamniocentesis AFP exceeded 19 micrograms/liter. Forty-eight (21.0%) of these displayed a significant elevation of AFP induced by the procedure, indicating fetal-maternal bleeding. This event was correlated with anterior placental location (P less than .025) and was followed by a significantly reduced mean fetal birth weight (3143 versus 3385 g, P less than .05). In addition, a nonsignificant (.10 less than P less than .20) doubling of the risk of giving birth to a child small for gestational age was observed. These findings suggest that amniocentesis represents a potential hazard to the fetus. The implications in relation to the widening indications for amniocentesis are discussed.