Poland has been known as an area with iodine deficiency. Surveys carried out in 1992/1993 and 1994 revealed that the voluntary model of iodine prophylaxis introduced in 1986 was ineffective. In 1997 a new model of iodine prophylaxis based on obligatory household salt iodization has been implemented. In order to assess its effectiveness new studies were undertaken in 1999-2001. The study involved 1471 school-children aged 6-15 years from 12 sites. In every subject thyroid volume by means of ultrasound and urinary iodine concentration were assessed. The results were compared with data obtained from the same schools in the 1992/1993 survey. Between 1992/93 and 1999/2001 goiter prevalence decreased from 14.5% to 5.2% (p<0.05) and median urinary iodine concentration increased from 56 microg/l to 103 microg/l (p<0.05). A decrease in goiter prevalence was observed in 6 sites with moderate goiter endemia, whereas the changes in goiter prevalence were statistically insignificant in other 6 sites. Three sites were characterized by goiter prevalence close to 5% before and after implementing the obligatory model of iodine prophylaxis. Goiter prevalence in the remaining three sites remained within the same limit of 7-10%, in spite of observed ioduria increase. The Authors conclude that the Polish model of obligatory iodine prophylaxis ensures efficient iodine supplementation and this is confirmed by a significant increment in ioduria. The effect of this model on thyroid volume is evident in moderate goiter endemia areas. Slight changes in goiter prevalence in mild goiter endemia regions need further monitoring and considering other factors affecting thyroid volume.