Monte Carlo field-theoretic simulations (MC-FTS) are performed on melts of symmetric diblock copolymer for invariant polymerization indexes extending down to experimentally relevant values of (N) over tilde similar to 10(4). The simulations are performed with a fluctuating composition field, W-(r), and a pressure field, W+(r), that follows the saddle-point approximation. Our study focuses on the disordered-state structure function, S(k), and the order-disorder transition (ODT). Although short-wavelength fluctuations cause an ultraviolet (UV) divergence in three dimensions, this is readily compensated for with the use of an effective Flory-Huggins interaction parameter, chi(e). The resulting S(k) matches the predictions of renormalized one-loop (ROL) calculations over the full range of chi N-e and (N) over bar examined in our study, and agrees well with Fredrickson-Helfand (F-H) theory near the ODT. Consistent with the F-H theory, the ODT is discontinuous for finite (N) over bar and the shift in (chi N-e)(ODT) follows the predicted (N) over bar (-1/3) scaling over our range of (N) over bar.