The severity of multiple sclerosis (MS) may change with the seasons, say US researchers.
Brain scans of patients compared with weather patterns at the time showed higher levels of disease activity in the spring and summer.
The US researchers said the findings had implications for testing new medicines, which may show up different results depending on the time of year.
It is not clear why warmer weather would have this 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.
For the study, researchers compared MRI brain scans of 44 people taken from 1991 to 1993 to daily temperature, solar radiation and precipitation measurements over the same time.
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.
Further analysis also showed that there was a link between both new disease activity and intensity of disease activity and the warmer 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.
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."
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."