We studied the effect of nitrate contamination of drinking water on volume and function of the thyroid in human populations exposed to different nitrate levels in their drinking water. Two sets of low and medium (tap) water, respectively medium and high (well) water nitrate exposure groups were compared. Drinking of nitrate-contaminated water was dose-dependently related with 24-h urinary nitrate excretion and salivary nitrate levels. No iodine deficiency was observed in any of the nitrate exposure groups. A dose-dependent difference in the volume of the thyroid was observed between low and medium vs. high nitrate exposure groups, showing development of hypertrophy at nitrate levels exceeding 50 mg/l. An inverse relationship was established between the volume of the thyroid and serum thyroid stimulating hormone (TSH) levels.