The sum of concentrations of inorganic arsenic, methylarsonic acid (MMAA) and dimethylarsinic acid (DMAA) in urine from adults and children living in an unpolluted area was compared with the corresponding sum measured in urine from adults and children living in an area polluted with arsenic and in urine from persons occupationally exposed to arsenic. The median values for 22 adults and 10 children aged 3-10 years living in the unpolluted area were 9.3 and 19.8 nmol As/mmol creatinine, respectively. The corresponding ranges were 3.2-27.9 and 7.7-57.8 nmol As/mmol creatinine, respectively. The difference between adults and children was tested by Mann-Whitney's non-parametric test and found to be significant (p less than 0.0025). No significant difference was found in arsenic concentration in urine from 73 adults and 10 children living in the polluted area compared with the arsenic concentration in urine from those living in the unpolluted area. The arsenic level in urine from adult workers handling arsenic-treated wood was approximately four-fold higher (p less than 0.001), with a maximum recorded concentration corresponding to 814.9 nmol As/mmol creatinine. The arsenic levels in urine from two glass workers were nine- and two-fold higher, respectively. No significantly increased arsenic levels were found in urine from workers impregnating wood and in urine from lead accumulator workers.