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3 dialyzers of each type were studied following dye injection into the dialysate compartment. Dynamic sequential imaging of longitudinal sections of the dialyzer were undertaken, using a new generation helical CT scanner (X-Press/HS1 Toshiba Corporation, Tokyo, Japan). In vivo studies: 3 dialyzers of each type were studied, in randomized sequence, in 3 different patients under standardized dialysis conditions. Blood- and dialysate-side urea clearances were measured at 30 and 150 minutes of treatment. Macroscopic and densitometrical analysis revealed that flow distribution was most homogeneous in the dialyzer with Moiré structure (Type C) and least homogeneous in the standard dialyzer (Type A). Space yarns (Type B) gave an intermediate dialysate flow distribution. Significantly increased urea clearances (p<0.001) were seen with Types B and C, compared to the standard dialyzer. Type C (Moiré) had the highest clearances although these were not significantly greater than Type B (space yarns). In conclusion, more homogeneous dialysate flow distribution and improved small solute clearances can be achieved by use of spacing yarns or waved (Moiré structure) patterns of fiber packing in the dialyzer. These effects are achieved probably as a result of reduced dialysate channeling resulting in a lower degree of mismatch between blood and dialysate flows. The new radiological technique using the helical CT scanner allows detailed flow distribution analysis and has the potential for testing future modifications to dialyzer design.