Previous studies have shown that warmer weather is associated with increased lesion activity in patients with multiple sclerosis (MS), and new research indicates that cognitive status is also temperature-dependent in people with this condition. In a study involving 40 patients with MS and 40 healthy controls, processing speed and memory were shown to decline on warmer days in the MS patients, but not in the controls. The authors suggest that cognitive status might provide a marker for MS exacerbation.