Many factors have been reported to have an influence in causing prematurity. We have reviewed the results of these previous studies and classified them in an attempt to find the most common predisposing factors. This review is preliminary to our own clinical and statistical investigation of the subject, the results of which will be published later.Some confusion has arisen from the difference in standards of determination of prematurity. The diagnosis is customarily based on the weight of the infant, even though the length may permit a closer estimate of maturity. A weight of 5 pounds (2,268 Gm.) has been used by some authors as the upper limit of prematurity and of 5½ pounds (2,500 Gm.) by others. Data collected under these two standards are not strictly comparable. In the following reviews and tables, no attempt has been made to designate or to separate the results according to the standards