Chromosomal aberrations in lymphocytes from peripheral blood were significantly more frequent in six workers from a plant manufacturing polyester resin boats (average 10.8 per 100 cells) than in six age-and sex-matched referents (5.2 per 100 cells). The contamination of the workroom air with styrene, as measured on three occasions within three years in different areas of the plant, was 50--400 mg/m3.