Chromosome analyses were carried out in lymphocytes of 32 workers occupationally exposed for more than 20 years to 50 Hz alternating electric and magnetic fields in 380 kV switchyards. As compared with a control group of 22 workers of similar age and occupation but without field exposure neither the yields of structural chromosome changes nor the SCE-frequencies were increased. The difference of cytogenetic data after occupational exposure to ionizing radiation is discussed.