Exposure to secondhand smoke (SHS) harms all children's health, especially children with asthma. Yet, children with asthma are as likely to live with smokers as healthy children. Household smoking bans are being advocated to reduce children's harm from SHS. To measure the effect of household smoking bans on child SHS exposure and to examine correlates of strict smoking bans in a low-income, diverse sample, 91 children with asthma were matched to 91 healthy children. All had at least one smoker living in their homes. Nicotine dosimeters, child cotinine assays, and maternal reports quantified child SHS exposures. Maternal reports of household smoking rules, behaviors, and beliefs, and other family characteristics were also gathered. The presence of a strict household smoking ban vastly reduced children's SHS exposures and was associated with fewer cigarettes smoked by the mother and by other family members, the belief that SHS was a personal health risk, having children with asthma, and living in a single-family home. Many children are exposed to high levels of SHS at home. Strict household smoking bans greatly decrease, but do not eliminate children's SHS exposure. Even in disadvantaged families, mutable factors were associated with strict smoking bans. Increased dissemination and use of established public health strategies are needed to reduce children's SHS exposures.