The effect of base curve and diameter variation on the fit or DuraSoft lenses was evaluated in terms of eight clinical fitting criteria: refraction, lens position, centering ability, stability of vision, amount and direction of movement, subjective comfort, and corneal physiological response. The minimum or critical intervals of base curve and diameter change necessary to produce significant changes in these criteria were determined. The results are discussed in relation to the design of diagnostic sets of lenses and the changes necessary to produce effective variations in clinical fit.