Background.- The pathophysiological relationship between episodic migraine a nd chronic migraine is not fully understood. We aimed to examine transcranial ma gnetic stimulation (TMS) indices of cortical excitability in patients with episo dic migraine (EM) and probable chronic migraine (PCM), and matched controls. Met hods.- Cortical excitability was assessed at baseline with two well- establish ed methods: phosphene thresholds (PT) and magnetic suppression of perceptual acc uracy (MSPA)- profiles. Five EM patients, five PCM patients, and five normal co ntrols participated in the main study. In addition, two patients were reassessed after 30 days of treatment with topiramate. Results.- Both PT and MPSA measure s were consistent in indicating a continuum of excitability across the three gro ups: PCM patients had the highest excitability, followed by EM, then controls. I n the two treated patients MPSA profiles appeared to normalize at a 100 mg dosag e. Conclusions.- Patients with PCM appear to be characterized by very high cort ical excitability. This may contribute to their greatly increased attack frequen cy. TMS- based methods will be important for future research examining the evol ution of chronic migraine from episodic migraine over time. Background- The Pathophysiological relationship between episodic migraine a nd chronic migraine is not fully understood. We aimed to examine transcranial ma gnetic stimulation (TMS) indices of cortical excitability in patients with episo dic migraine (EM) and probable chronic migraine (PCM), and matched controls. Met hods.- Cortical excitability was assessed at baseline with two well-established ed methods: phosphene thresholds (PT) and magnetic suppression of perceptual acc uracy (MSPA) - profiles. Five EM patients, five PCM patients, and five In addition, two patients were reassessed after 30 days of treatment with topiramate. Results.- Both PT and MPSA measure s were consistent in indicating a continuum of excitability across the three gro ups: PCM patients had the highest excitability, followed by EM, then controls. I n the two treated patients MPSA profiles appeared to normalize at a 100 mg dosag e. Conclusions .- Patients with PCM ap pear to be characterized by very high cort ical excitability. This may contribute to their greatly increased attack frequen cy. TMS-based methods will be important for future research examining the evol ution of chronic migraine from episodic migraine over time.