Timing of first appearance and subsequent distribution of adipose tissue were assessed in 488 normal-for-age human fetuses. The sample represented each of the three trimesters of normal pregnancies. Light microscopy showed that adipose tissue first appears and progressively develops from the 14th to 24th week of gestation (100-216 mm crown-rump length) in those areas where it characteristically accumulates after birth. No significant sex differences were found in patterns of early fat deposition. It is suggested that the second trimester of gestation is the critical or key period in fat adipogenesis.