Fifty-four subjects volunteered to participate in a controlled study contrasting spinal manipulation with spinal mobilization without the rotational forces and leverage required to move facet joints. All suffered from regional low-back pain for less than 1 month, were ages 18-40, had never previously undergone any form of spinal manipulation, and denied a prior episode of backache within the previous 6 months. Randomization was stratified at outset into those who suffered for less than 2 weeks and those whose discomfort had persisted for 2-4 weeks. Outcome was monitored by a questionnaire assessing functional impairment. A treatment effect of manipulation was demonstrated only in the strata with more prolonged illness at entry. In the first week following manipulation, these patients improved to a greater degree (P = .009, t test) and more rapidly (P less than .025, Wilcoxon rank-sum test).