The impact of multiple sclerosis (MS) could change with the seasons, say US researchers. It turns out to be more severe in spring and summer, they say after a study of MRI brain scans.
It is not clear though why warmer weather should have such an effect.
Other studies have shown that vitamin D from exposure to sunlight may have a protective effect against MS - a long-term inflammatory condition of the central nervous system.
The adults in the study, who had untreated MS, had eight weekly scans followed by eight scans every fortnight then six monthly check-ups - an average of 22 scans per person.
After one year, 310 new brain lesions were found in 31 people, they reported in Neurology.
The lesions were up to three times more likely to appear in the warmer spring and summer months.
Study leader Dr Dominik Meier, from Brigham and Women's Hospital in Boston, said: "Not only were more lesions found during the spring and summer seasons, our study also found that warmer temperatures and solar radiation were linked to disease activity."
He pointed out that clinical trials often use MRI (magnetic resonance imaging) to assess the effectiveness of a drug and studies commonly last between six and 12 months, which may have implications for how effective a new medication seems.
The research was published in Neurology, the medical journal of the American Academy of Neurology.
In an accompanying editorial Dr Anne Cross, from Washington University School of Medicine, added: "This is an important study because it analyses records from the early 1990s, before medications for relapsing MS were approved, so medicines likely could not affect the outcome.
"Future studies should further explore how and why environmental factors play a role in MS."
This research could have a large impact on the testing of new medicines, as the six-month trials could show different results depending on which season they are performed in.
Dr Susan Kohlhaas, research communications officer at the MS Society, said more research was needed.
But added: "This small study is intriguing and, if validated in larger studies, has the potential to influence the way clinical trials are designed
Multiple sclerosis is an autoimmune disease that impacts the central nervous system and affects approximately one in 1,000 individuals in North America.