Several studies have claimed the effectiveness of repetitive transcranial magnetic stimulation (rTMS) in Parkinson's disease (PD). The rTMS therapy has to be repeated regularly to achieve a permanent effect but the side effects of long-term administration of low frequency rTMS are not known. Further, there is no information about its influence on the development of Parkinson's disease. Two different groups of patients with PD were compared in a retrospective study for 3 years. The first group (A) was treated with drugs, the second group (B) was treated with drugs + rTMS (1 Hz, 0.6 T, 100 stimuli per day for 7 days using a round coil). rTMS was repeated at least twice each year for 3 years. Symptoms of PD were assessed using the Graded Rating Scale. Although at the onset of the study group B patients had greater disease severity and were receiving higher doses of levodopa, this group (receiving rTMS) showed no deterioration in these parameters, whereas those in group A receiving drugs alone showed a marked deterioration. Hoehn-Yahr (H-Y) stages at the onset of the study and 3 years later were: group A: 1.93 +/- 0.75, 3.03 +/- 1.01; group B: 2.50 +/- 0.83, 2.45 +/- 0.62. The dose of levodopa (mg/day) was at the onset of trial and 3 years later was: group A: 124.4 +/- 144.0, 555.5 +/- 247.2; group B: 287.7 +/- 217.1, 333.4 +/- 181.0. The yearly increment in the scores was: group A: 1.308 +/- 0.307 (P < 0.001), group B: 0.642 +/- 0.389 (P < 0.1). Accordingly, this retrospective study using regularly repeated rTMS with 1 Hz for 7 days, at least twice yearly for 3 years, significantly slowed the development of Parkinson's disease. Unwanted side effects were not observed during the 3 years.