Solid holdup in vertical fluidized beds for liquid-solid systems was investigated over a wide range of experimental conditions. The solid phases used were sieved glass beads of diameters 66, 118, 243, 465, 900, 2200, 3100 and 4650 mu m, sieved porous alumina particles of diameter 3680 mu m and sieved ion exchange resin particles of diameter 610 mu m. The liquid phase was tap water. The effect of column diameter on solid holdup was negligible when the column diameter was larger than 0.0397 m. A novel model for predicting solid holdup in all flow regimes was developed. Experimental data, including previous results in the literature, were in good agreement with the model.