We have conducted detailed comparisons of the predictions of self-consistent mean-field (SCMF) theory with results of Monte Carlo simulations for bulk lamellar structures of symmetric diblock copolymers A-B in concentrated solutions. Large differences are found among the values of the Flory-Huggins parameter extracted in different ways. Under the convolution approximation, we assess the accuracy of the SCMF theory by comparing simulated lamellar profiles with those obtained from the theory. A Flory-Huggins parameter inferred from the free energy of the system provides good overall agreement between simulated and theoretical lamellar profiles. The results of simulations for chain dimensions scale with the product is an element ofN, where is an element of denotes the repulsion between A and B segments, and N is the chain length.