Seventeen subjects sustained focus for up to 8 min on targets placed either at the nearpoint (NP) or farpoint (FP) of accommodation. This was repeated in separate sessions from three to six times at each target distance and pre- to postexposure changes in dark focus (DF), i.e., accommodative hysteresis, were recorded. For eight subjects the probability (P) of exhibiting hysteresis effects was greater than or equal to 0.50 on both types of sessions. Five additional subjects showed P(NP) greater than or equal to 0.66 and P(FP) less than or equal to 0.33, whereas 4 subjects showed a reversed preference with P(NP) less than or equal to 0.17 and P(FP) greater than or equal to 0.83. Thus, strong individual differences emerged with respect to susceptibility to accommodative hysteresis. Linear regression analyses were performed on the individual data of the 13 subjects in condition NP and 12 in condition FP who exhibited hysteresis effects on greater than or equal to 50% of their sessions. Relative magnitudes of accommodative hysteresis resulting either from sustained NP or FP focusing were negatively related to the DF level measured directly before the period of sustained focus. Thus, for example, a relatively near initial DF correlated with a small inward shift of the DF after NP focusing, but with a large outward shift after FP focusing. Only in the FP condition was there evidence for a positive regression of hysteresis effects on the dioptric distance separating the sustained focus target and the initial DF level, viz., the less the separation, the smaller the hysteresis effect.(ABSTRACT TRUNCATED AT 250 WORDS)