The present study investigated the ad libitum eating response of dieters and non-dieters after (a) no preload, (b) a large preload (15 oz. milkshake), or (c) an extra-large preload (30 oz. milkshake). Non-dieters ate progressively less following larger preloads, exhibiting normal compensation. Dieters, however, ate more ad libitum ice cream after the large preload than after no preload, thus exhibiting the well-established "counter-regulation" effect. After the extra-large preload, dieters ate very little ice cream ad libitum, suggesting that they are responsive to satiety pressures. These results were interpreted as indicating that dieters' ad libitum eating is neither unregulated nor truly counter-regulated. We demonstrate that dieters eat more after a large preload than after a small preload (i.e. exhibit "counter-regulation") only under certain clearly specifiable conditions. Consideration of our "boundary model" helps in understanding whether regulation or counter-regulation is likely to be observed in dieters.