Fluctuation effects in symmetric diblock copolymer melts are quantitatively examined with the first direct comparison between Monte Carlo simulations and mean-field theory where both are performed on the same identical model. The simulations provide the most conclusive evidence to date that fluctuations transform the continuous mean-field order-disorder transition (ODT) into a discontinuous transition. Furthermore, the fluctuations shift the ODT toward high segregation, chiN, by a factor of similar to2.7, which is much greater than that suggested by the Fredrickson-Helfand fluctuation theory. For the range of small molecules examined in our study, this shift is nearly independent of molecular weight. In the disordered phase, we find the usual large deviations from mean-field theory, but we find no indication whatsoever that they set in abruptly at some well-defined temperature as previously claimed. (C) 2003 American Institute of Physics.