Since publication of the paper by Bard et al. (1990) it has been known that GCM studies of the climate of the last glacial maximum (LGM), employed lower boundary conditions appropriate to this time but astronomical parameters of an era 3000 years later. The LGM boundary conditions from CLIMAP were for 18 kyr BP on the 14C timescale. The GCM simulations employed the insolation regime appropriate to 18 kyr BP on the side-real timescale whereas the appropriate LGM insolation regime is that of the 21 kyr BP. These studies also used the CLIMAP ice sheet reconstruction. However, on the basis of most recent analyses the reconstruction by Tushingham and Peltier (1991) is to be preferred. Hyde et al. (1989) showed that a simple EBM compared favorably with the NCAR CCM, when both were used to simulate the temperature distribution of the LGM. Here we shall employ the same EBM to study the effect on LGM climate of the timing mismatch, and of the different horizontal extents of the different ice sheet reconstructions. In each case the climatic effect is found to be significant. Thus we cannot claim an accurate LGM simulation unless the orbital and terrestrial inputs match to within 1000 years and unless we employ the best possible ice sheet reconstruction in the analysis. ¿ American Geophysical Union 1993