This work describes some of the limitations encountered when sedimentation/steric field-flow fractionation is used to separate and analyze particles whose diameters approach a substantial fraction of the thickness of the FFF channel. Results obtained for a thin (127 mu m thick) channel indicate a departure from linear calibration for particle diameters that approach 25-30% of the channel thickness and a substantial loss of selectivity for diameters at 50% of the channel thickness. These results are explained by a simple theoretical model. In addition, our measurements show that the larger particles (in this case polystyrene latex beads of 60 mu m diameter) are subject to peak distortion and sample loss unless high flowrates and low field strengths are employed.