This study inquires if there is an increase of chromosomal aberrations by ionising radiation of cosmic origin in civil pilots and flight-crew members. Totals of 37,208 exposed cells and 10,950 control cells, from 192 and 55 donors respectively, were scored averaging 200 observations per subject. The analysis showed the increase of dicentric and ring chromosomes in peripheral blood lymphocytes of the flight personnel. The difference of the total aberration frequencies between exposed and control was statistically significant.