We have previously suggested that patients with unilateral neglect may be limited in their ability to sequentially attend or act upon stimuli. To assess the nature of this capacity limitation, we examined the relationship between number of stimuli presented on cancellation arrays and how many targets a patient with neglect cancelled. This relationship was systematic and described by a power function: targets cancelled=K (targets presented)B, in which the constant and exponent were derived empirically. Density of targets and time taken to cancel targets did not account for the relationship. Improvement on subsequent testing was reflected in an increase in the constant. However, the exponent of the power function did not change, suggesting that some critical aspect of her dysfunction remained the same. These data also imply that she had implicit knowledge of quantity of stimuli presented, and that this knowledge systematically influenced her explicit behavior. © 1992.