High prevalence of goiter, other IDD such as impaired physical and intellectual growth and hearing deficit have been reported previously in Kiga. In order to evaluate the effect of iodized oil injection, this study was conducted in schoolchildren of Kiga village from 1989 to 1992. One ml of iodized oil solution containing 480 mg of iodine was injected into 198 schoolgirls and boys aged 8-14 years. Serum thyroid hormones, RT 3 U, TSH and thyroglobulin, before and 12, 24 and 36 months after the intra-muscular injection of iodized oil were measured. Assessment of urinary iodine was performed at the same periods by Foss method. Prior to the injection, all schoolchildren had goiters larger than grade 1 A (48% were grade 3); 3 years after intervention 20% had grades zero and 1 A and 8% grade 3 (P < 0.001). Urinary iodine was 11.4 +/- 19.8 before and increased to 113 +/- 63 and 83 +/- 66 microg/g creatinine 2 and 3 years after intervention. Mean serum T 4 was 5.0 +/- 2.1, 10.8 +/- 2.8, 9.8 +/- 2.5 and 9.5 +/- 2.1 microg/dl before and 12, 24 and 36 months after the injection, respectively (P < 0.001). Mean serum TSH was 20.3 +/- 22.8, 1.2 +/- 1.6, 0.8 +/- 1.2 and 2.2 +/- 0.9 mU/L in the same intervals, respectively (P < 0.001). Mean serum thyroglobulin was 132 +/- 107, 10 +/- 12 and 23 +/- 20 ng/ml before and at 2 and 3 years after injection, respectively (P < 0.001). Slight but significant increases in serum TSH and thyroglobulin occurred at 3 years after the injection. Findings show benefits of iodized oil administration in decreasing goiter size and in resuming normal thyroid function up to 3 years after the intervention. An increase in TSH and or thyroglobulin could be considered as the first sign of a fall in effectiveness of iodized oil injection.