Over a 6-month period, five patients with early relapsing--remitting multiple sclerosis and five with long-standing, benign multiple sclerosis underwent serial psychometric testing and contrast enhanced magnetic resonance imaging of the brain at 2-weekly or monthly intervals, respectively. All patients were individually matched with healthy controls who completed the same psychometric battery at the same time intervals. As a group, multiple sclerosis patients either made more errors or performed slower on all psychometric tasks than controls. In the control subjects and those patients with a stable brain lesion score, no consistent deterioration occurred in any test and the overall pattern was one of improvement over time commensurate with practice effects. However, patients with a deteriorating lesion score either showed a fall-off in performance on some psychometric tasks (patients 2, 3) or else an impaired ability to improve with practice on certain tests of attention and information-processing speed (patient 10).