Urinary iodine (I) and serum and urinary thiocyanates (SCN-) were determined in sample groups from 12 areas of endemic goiter in Italy. The mean urinary I level of 637 subjects with thyroid 0 was 67 +/- 31 (mean +/- SD) microgram/liter, that of 648 with goiter 54 +/- 29 micrograms/liter. Mean serum and urinary SCN- were 2.44 +/- 1.36 mg/liter and 2.58 +/- 1.36 mg/liter in 887 and 1531 subjects, respectively. The thiocyanates data enabled a distinction to be drawn between two groups (populations). Population I included 73% of the adults and 92% of the school-children (6-16 yr). Its thiocyanate values were logarithmically spread around means of 1.24 +/- 0.6 mg/liter (serum) and 1.24 +/- 0.57 mg/liter (urine), whereas those of population II were widely dispersed around means of 6.1 +/- 3.0 mg/liter and 8.08 +/- 5.5 mg/liter respectively. The boundary between the two populations was set at 3 mg/liter urine SCN-. This distinction was substantiated by the fact that 90% of those in population II smoked 10 or more cigarettes a day, whereas population I comprised occasionally smokers only. It is believed that only the values in population I can be regarded as representative of thiocyanates either endogenous or due to dietary dietary intake: these values never differed more than 60% regardless of the areas, seasons, or dietary habits. As urinary SCN- levels rose, there was also an increase in urinary iodine excretion within certain limits. This, however, did not interfere with thyroid secretion. The urinary I/SCN- ratio was lower in subjects with goiter. This was because their iodine levels were lower, whereas SCN- values were much the same in subjects with and without goiter. We have found no correlation between thiocyanate itself and goiter.