© 2015, BMJ Publishing Group. All rights reserved. Objective Healthcare and mortality costs of secondhand smoke (SHS) exposure at home among nonsmokers in California were estimated for the year 2009. Methods Costs were estimated with an epidemiological model using California SHS home exposure rates and published relative risks. Healthcare costs included nine conditions, and mortality was estimated for four perinatal and three adult conditions. Three mortality-related measures were estimated: deaths, years of potential life lost (YPLL) and the value of lost productivity. Results SHS-attributable healthcare costs totalled over $241 million. The most costly conditions for children and adolescents were attention deficit hyperactivity disorder ($7.8 million) and middle ear disease ($5.6 million). For adults, the most costly conditions were ischaemic heart disease (IHD) ($130.0 million) and asthma ($67.4 million). Deaths of 821 Californians were attributable to SHS exposure in the home, including 27 infants whose mothers smoked while pregnant and 700 adults who died from IHD. These deaths represented a loss of over 13 000 YPLL and $119 million in lost productivity. Conclusions The economic impact of SHS exposure in the home totalled $360 million in California in 2009. Policies that reduce exposure to SHS at home have great potential for reducing healthcare and mortality costs.