The initial effectiveness as well as the temporal stability of the effect of cervical spinal manipulation with respect to the amelioration of goniometrically verified cervical lateral-flexion passive end-range asymmetry was examined. Responses of two groups of pain-free subjects were compared: a) those exhibiting end-range asymmetries of greater than 10 degrees who, in addition, had suffered previous neck trauma, and; b) those who happened to exhibit end-range asymmetries of greater than 10 degrees but who had no history of prior neck trauma. All subjects received a single lower cervical adjustment delivered to the side of most-restricted end-range, and goniometric reassessments were performed 30 min, 4 hr, and 48 hr following the adjustment. A dramatic amelioration of asymmetry was observed in both groups at 30 min and 4 hr postmanipulation. Furthermore, the magnitudes of these short-term effects were similar for the two groups. However, by 24 hours, a difference in the temporal responses of the groups had become readily apparent. By 48 hours, the difference was even more striking; whereas 14 of 16 of the subjects with no previous neck trauma continued to exhibit asymmetries of less than 10 degrees (mean +/- SEM = 3.8 +/- 1.0 degrees), 12 of the 16 subjects with previous neck trauma had regained asymmetries of greater than 10 degrees (mean +/- SEM = 11.4 +/- 1.7 degrees). These results indicate that among asymptomatic (pain-free) individuals, the mere presence of passive end-range asymmetry as well as the magnitude of the short-term ameliorative effect of cervical manipulation do not distinguish these two categories of subjects.(ABSTRACT TRUNCATED AT 250 WORDS)