Contrast thresholds were measured for sinusoidal gratings, with Gaussian spatial and temporal envelopes, as a function of spatial extent in the presence and absence of dynamic white noise. Spatial frequencies ranged from 0.5 to 32 c/deg. Efficiency is defined as the ratio of the ideal's contrast-energy threshold to that of the observer under study, each at the same performance level. For spatial frequencies from 0.5 to 8 c/deg, efficiencies for gratings in noise ranged from 8 to 30% for widths less than one cycle, but dropped rapidly as the width was increased beyond one cycle. Spatial summation of gratings in noise resembles the performance of a cross-correlator whose template is matched to a signal about 1 cycle wide (between 1/e points). The psychometric function slope is consistent with this idea.