The objective of this prospective study was to determine if a correlation could be established between the ground-level concentrations of sulfur dioxide and methemoglobin concentrations in pregnant women when a coal-powered thermoelectric power plant was in operation ("dirty" period) and when it was closed ("clean" period). The location of the power plant, Plomin 1, in Labin, Croatia, was taken into consideration. Blood and urine samples of each pregnant woman in the study were tested three times in the clean period (n = 138) and three times in the dirty period (n = 122), with 1 month between each test. I observed a correlation between the increase in mean values of methemoglobin and the ground-level concentration of SO(2) on corresponding dates during the dirty period (r = 0.72, p < 0.01). In the clean period, the negative mean value of methemoglobin was significant (r = -0.60, p <or=0.05), whereas in the dirty period, the positive mean value of methemoglobin was significant (r = 0.73, p 0.01). The increase of maternal methemoglobin could be a useful biomarker to determine when the health of pregnant women is threatened by toxic substances in the environment.