Rod system increment threshold functions (ITFs) were studied in patients with cone-rod dystrophy (CRD). Rod thresholds (I(t)) for a 104-min, 500-nm test stimulus (TS), superimposed on an 11 degrees long wavelength background (luminance I(b)), were measured in eleven CRD patients and fourteen normal subjects. Thresholds in normals were also measured using a 7-min TS. A modified version of Weber's law [logI(t) = logK + log(I(n)(b) + c)] was fitted to the data from each subject and test condition to yield a description of the ITF in terms of the free parameters K, c, and n. Four of the CRD patients exhibited an abnormally high absolute threshold; of these, two showed abnormalities in K and c consistent with a reduced efficiency of quantum capture by the rods. Abnormalities in the ITFs of CRD patients did not resemble the effect of reducing the diameter of the TS from 104 to 7 min in normals. This suggests that threshold abnormalities in the CRD patients did not result from altered spatial summation. The results illustrate use of a parametric representation of the ITF to evaluate the loss of sensitivity in visual disorders.