Naturally occurring isotopes of radon in indoor air are identified as the second leading cause of lung cancer after tobacco smoking. Winnipeg had the highest radon ((222)Rn) concentration among 18 Canadian cities surveyed in the past. There is great interest to know the current radon as well as thoron ((220)Rn) concentrations in Winnipeg homes. Therefore, radon-thoron discrimination detectors were deployed in 117 houses for a period of 3 months. The results confirmed that thoron is present at detectable levels in about half of the Winnipeg homes and radon remains significantly higher than the national average. In this study, radon concentrations ranged from 20 to 483 Bq m(-3) with a geometric mean of 112 Bq m(-3) and a geometric standard deviation of 2.07. It is estimated that 20% of Winnipeg homes could have radon concentrations above the Canadian indoor radon guideline of 200 Bq m(-3). This conclusion is similar to the previous estimation made 20 y ago. Thoron concentrations were below the detection limit in 60 homes. Among the homes with detectable thoron concentrations, the values varied from 5 to 297 Bq m(-3), the geometric mean and standard deviation were 21 Bq m(-3) and 2.53, respectively.