Study Design. A prospective randomized trial in 97 patients with a whiplash injury caused by a motor vehicle collision. Objectives. The study evaluates early active mobilization versus a standard treatment protocol and the importance of early versus delayed onset of treatment. Summary of Background Data. There is no compelling evidence to date on the management of acute whiplash-associated disorders. The few studies describing treatment, however, provide evidence to support the recommendation that an active treatment in the acute stage is preferable to rest and a soft collar in most patients. Methods. Patients were randomized to four groups. Active versus standard treatment and early (within 96 hours) versus delayed (after 2 weeks) treatment. Measures of range of motion and pain were registered initially and at 6 months. Results. Eighty-eight patients (91%) could be followed up at 6 months. Active treatment reduced pain more than standard treatment (P < 0.001). When type and onset of treatment were analyzed, a combined effect was seen. When active treatment was provided, it was better when administered early, and if standard treatment was provided, it was better when administered late for reduction of pain (P = 0.04) and increasing cervical flexion (P = 0.01). Conclusions. In patients with whiplash-associated disorders caused by a motor vehicle collision treatment with frequently repeated active sub maximal movements combined with mechanical diagnosis and therapy is more effective in reducing pain than a standard program of initial rest, recommended use of a soft collar, and gradual self-mobilization. This therapy could be performed as home exercises initiated and supported by a physiotherapist.