This study was undertaken to determine the concentrations of formaldehyde, acetaldehyde, and acrolein in air samples taken in some Canadian houses and to determine the association between aldehyde levels and housing characteristics. Concentrations of formaldehyde, acetaldehyde, and acrolein were measured in 59 homes in Prince Edward Island, Canada, during the winter of 2002. Housing characteristics were documented through inspection and by interviews of occupants. Formaldehyde, acetaldehyde, and acrolein concentrations ranged from 5.5 to 87.5 microg/m(3) (median, 29.6 microg/m(3)), from 4.4 to 79.1 microg/m(3) (median, 18.9 microg/m(3)), and from 0.1 to 4.9 microg/m(3) (median, 0.9 microg/m(3)), respectively. Formaldehyde levels were elevated in homes built after 1970. Acetaldehyde and acrolein levels were elevated in homes inhabited by at least one smoker and in homes built 1970--1985 and were correlated with absolute humidity and carbon dioxide, two variables likely to be surrogates for lower air exchange rates. In conclusion, lower air exchange rates appear to be important determinants of formaldehyde, acetaldehyde, and acrolein levels in homes. These data also confirm that smoking is a significant source of acetaldehyde and acrolein and indoor air.