They studied 24 stroke survivors with aphasia, 13 of whom were given 20 minutes of low-level brain stimulation using a hand-held magnetic coil. Eleven were given a sham treatment.
Each patient then received 45 minutes of speech and language therapy, and the same regime was repeated daily for 10 days.
On average, the patients who underwent the brain stimulation each day made 20 to 30 per cent more improvement, researchers reported in the Stroke journal.
Dr Alexander Thiel, who led the study, said: "This is similar to physical rehabilitation where the unaffected limb is immobilised with a splint so that the patients must use the affected limb during the therapy session.
"We are entering exciting times where we might be able in the near future to combine speech and language therapy with non-invasive brain stimulation earlier in the recovery. This could result in earlier and more efficient aphasia recovery and also have an economic impact."