The concentration of phthalic acid was determined in the urine of nine subjects occupationally exposed to phthalic anhydride. For the determination, the urine samples were acidified, extracted with dimethyl ether, esterified with boron trifluoride/methanol and measured by electron capture gas chromatography. Environmental air samples were collected in Tenax tubes, eluted with methyl-t-butyl ether and assayed by electron capture gas chromatography. Significant correlations were found between the concentration in urine samples (range: 0.3-14.0 mumol/mmol creatinine), collected at different times of the day, and the time-weighed average atmospheric concentrations (range: 0.03-10.5 mg/m3). No conjugation of phthalic acid in the urine was observed. The detection limit for urine samples (10 ml) was 0.05 mumol/l) and that for air samples 0.4 microgram/m3. The method has potential for biological monitoring of workers exposed to phthalic anhydride. It was found that at atmospheric anhydride concentrations of about 30% of the hygienic reference value (6 mg/m3), which is applied in many market economies, a body-burden was caused which was not eliminated overnight.