In Sarawak, East Malaysia, studies have found a high prevalence of IDD in some locations. This study investigates the relationship between iodine status and mental development among young children in Sarawak. 3 districts: Kuching, Bau and Simunjan were chosen based on advice from the Health Authority in Sarawak and these correspond with an IDD prevalence that is lowest in Kuching, intermediate in Bau and highest in Simunjan. 803 school children from 19 schools were selected via proportionate systematic sampling in the 3 districts. Urine samples were tested for iodine and creatinine. The colorimetric ceric-arsenic assay was applied to determine iodide content and the Jaffe method with deproteinization was used to determine creatinine level. Half of the children were from Kuching and 22% and 24% were from Bau and Simunjan districts respectively. Almost all were evenly distributed by sex. By mother's race, almost half were Malays, followed by Bidayuh, Iban, Chinese and other races. The prevalence rate of goitre was low at 0.5%. A discrepancy in iodine, creatinine and iodine/creatinine ratio was shown between the districts, where the more developed area like Kuching has a highest value. These levels were also significant in terms of race and household income. The levels were high for Malay mothers and those having higher household income. On the mental performance measured by the Ravens test, the total scores were significantly different by district, mother's race, education and household income. More developed district like Kuching, higher household income and mothers who are better educated tend to have a higher Raven's total score. This trend is similar to those iodine-related indicators and could be accounted for by the infrastructure, socio-economic development and nutritional practices in these districts.