The use of assisted reproductive technology is increasing rapidly. Research, although sparse, has resulted in inconsistent findings as to the developmental prognosis for infants conceived by assisted reproductive techniques such as in vitro fertilization and the use of fertility drugs. In the present study, the authors compared twins who were spontaneously conceived with those who were conceived through assisted reproductive technology. The authors found differences in birth weight and gestational age. Infants conceived by assisted reproductive technology fared worse than did those who were spontaneously conceived. The authors found no differences between the groups in mental development at 24 months of age, but they found evidence of differences in physical development. Implications of the findings are discussed. (Contains 1 table.)