To evaluate the habits of betel quid use and areca nut chewing among school-aged children in Karachi, Pakistan. Areca nut (betel nut) is chewed by itself, in various scented preparations, and in betel quid (containing betel leaf, areca nut, slaked lime, condiments, sweeteners and sometimes tobacco) in various parts of Pakistan and India. It is associated with carcinogenesis, foreign body aspiration in children and oral submucous fibrosis, and may aggravate asthma. We selected a stratified random sample of 160 primary school children between 4 and 16 years of age in Baba Island, Karachi. Seventy-four per cent of the children (118/159) used areca nut and 35% (55/159) used betel quid daily. More boys chewed areca nut than girls (72% vs 30%). The proportion of areca nut users increased by grade (from 48% in first grade to 90% in fifth grade). Most areca users first tried it with a family member (42%) or a friend (26%), and most (68%) consumed three or more packets a day. Children with fathers with three or fewer years of education were more likely to use areca nut (OR 3.2; 95% CI 1.2-8.4), and children whose mothers helped with homework less likely (OR 0.5; 95% CI 0.2-0.91; P = 0.027) to use it. Boys (OR 6.6; 95% CI 2.3-18.7) and areca nut users (OR 8.8; 95% CI2.8-27.0) were more likely to use betel quid. To reduce the use of areca nut, the Pakistan Government should consider imposing taxes on it, limiting advertising and actively communicating its health risks to the public.