We describe the association between structural chromosome aberrations (CAs) and parameters of exposure to arsenic among 42 individuals exposed to arsenic through well waters in Finland. The median concentration of arsenic in the wells was 410 microg/l, the total arsenic concentrations in urine (As-tot) was 180 microg/l, and in hair 1.3 microg/g, for current users (n = 32) of contaminated wells. Urinary arsenic species and CAs were also analyzed in eight control individuals from the same village who consumed water which contained arsenic <1.0 microg/l (detection limit). Increased arsenic exposure, indicated best by increased concentrations of arsenic species (inorganic arsenic, methylarsonic acid (MMA), dimethylarsinic acid (DMA)) in urine, was associated with increased frequency of CAs. The increased urinary ratio of MMA/As-tot and the decreased ratio of DMA/As-tot were associated with increased CAs when all aberration types, including gaps, were considered. Associations between CAs and arsenic exposure indicators were stronger among current users than among persons who had stopped using the contaminated well water for 2-4 months before sampling (ex-users, n = 10). Furthermore, there was a positive but not statistically significant association between CAs and arsenic in hair among the current users, but not among the ex-users, who still had relatively high arsenic concentrations in hair. The results suggest that the effect observed in the present study reflects relatively recent arsenic exposure.