Accumulating evidence indicates that energy-yielding beverages evoke weaker appetitive responses than more solid food items, but the properties responsible have not been characterized. The present study attempted to isolate an influence of viscosity. At weekly intervals, 84 adults ingested 325-ml (220 kcal) shakes that were matched on weight, volume, temperature, energy, macronutrient content, energy density, rate of consumption, cognitive expectations, palatability, appearance, and requirements for mechanical processing, but varied in viscosity. Twice appetitive ratings were obtained over the subsequent 4 h, while all intake was proscribed, and twice ratings were kept until the first spontaneous eating occasion comprised of > or =100 kcal. Dietary intake was recorded over the 24 h after shake ingestion. Significantly greater and more prolonged reductions of hunger were observed with the thicker shake. No significant differences were noted in the size or time to first meal or 24 h energy intake. These data indicate viscosity exerts an independent inverse effect on hunger in humans.