A deficiency of iodine is characteristic of mountainous regions of the world, and the frequency of goitre in such areas has been recognized for centuries. The aim of the present study was to estimate the prevalence of goitre among schoolchildren in high and low altitude areas of Asir Region. The study was carried out upon 940 male students randomly selected from 12 schools at 2 high altitude areas (Tamnia and Al-Soda) 3150 meters above sea level and one low altitude area (Marabah) 500 meters above sea level. All students were subjected to clinical examination of the thyroid and classification of goitre grading was based on the criteria endorsed by the World Health Organization/United Nations Children's Fund/International Council for the Control of Iodine Deficiency Disorders. An overall prevalence of goitre of 24% was estimated in the areas studied (24% in elementary, 24% in intermediate and 23% in secondary education). This prevalence was significantly higher (p < 0.0001) in high altitude (27%, 95% confidence interval: 24%-30%) than in low altitude areas (13%, 95% confidence interval: 8%-18%). Children of high altitudes were 2.5 times more likely to develop goitre as compared to their counterparts in low altitudes (odds ratio = 2.5, 95% confidence interval 1.6-3.8). However, comparison between the 2 high altitude areas revealed that children of Tamnia area were nearly 2 times more likely to develop goitre than their counterparts in Al-Soda (odds ratio = 1.9, 95% confidence interval: 1.3-2.6). Endemic goitre is more prevalent in mountainous, high altitude areas of Asir Region. However, the distribution of goitre in these areas is patchy and differs from area to area. Well water might have been a contributing factor for the high prevalence in Tamnia.