Objective: Urinary iodine concentration is the prime indicator of nutritional iodine status and is used to evaluate population-based iodine supplementation. In 1994, WHO, UNICEF and ICCIDD recommended median urinary iodine concentrations for populations of 100-200 μg/l, assuming the 100 μg/l threshold would limit concentrations <50 μg/l to ≤20% of people. Some scientists felt this proportion was unacceptably high and wanted to increase the threshold above 100 μg/l. The study was carried out to determine the frequency distribution of urinary iodine in iodine-replete populations (schoolchildren and adults) and the proportion of concentrations <50 μg/l. Method: A questionnaire on frequency distribution of urinary iodine in iodine-replete populations was circulated to 29 scientific groups. Findings: Nineteen groups reported data from 48 populations with median urinary iodine concentrations >100 μg/l. The total population was 55 892, including 35 661 (64%) schoolchildren. Median urinary iodine concentrations were 111-540 (median 201) μg/l for all populations, 100-199 μg/l in 23 (48%) populations and ≥200 μg/l in 25 (52%). The frequencies of values <50 μg/l were 0-20.8 (mean 4.8%) overall and 7.2% and 2.5% in populations with medians of 100-199 μg/l and >200 μg/l, respectively. The frequency reached 20% only in two places where iodine had been supplemented for <2 years. Conclusion: The frequency of urinary iodine concentrations <50 μg/l in populations with median urinary iodine concentrations ≥100, μg/l has been overestimated. The threshold of 100 μg/l does not need to be increased. In populations, median urinary iodine concentrations of 100-200 μg/l indicate adequate iodine intake and optimal iodine nutrition.