The effect of prolonged iodine deficiency on learning and motivation was studied. One hundred male children--matched for age, socioeconomic status, and formal education--were selected from both severely iodine-deficient (SID) and mildly iodine-deficient (MID) villages. Mean urinary iodine excretion was significantly lower in the SID than in the MID group (219.84 +/- 57.52 compared with 449.14 +/- 32.31 nmol/L, P < 0.001). The serum thyroxine concentration was significantly lower (90.36 +/- 6.46 compared with 123.70 +/- 15.42 nmol/L, P< 0.001) and serum thyroid-stimulating hormone (TSH) was significantly higher in the SID group than in the MID group (6.23 +/- 0.34 compared with 4.85 +/- 0.28 mU/L, P< 0.01). The children were administered maze, verbal, and pictorial learning tasks and a test of motivation. The results showed that SID children are slow learners compared with MID children. In both groups the rate of learning over trials was superior in younger (aged 9-12 y) children although the initial performance of older (aged 12-15 y) children was better (P < 0.01). SID children scored significantly lower than MID children on the achievement motivation scale (P < 0.01). The results are suggestive of neural impairment as well as poor sociopsychologic stimulation, resulting in learning disability and lowered achievement motivation. Unless iodine nutrition is improved in the community as a whole, these abnormalities may prevent millions of children from the SID areas from achieving their full potential even if learning opportunities are made available to them.