The format in which a probability is presented and a person's numeracy can influence comprehension of health risk probabilities (e.g., Galesic, Gigerenzer, & Straubinger, 2009). Many people, especially older adults, have inadequate numeracy (Kutner, Greenberg, & Baer, 2005), which may interact with comprehension of different formats (e.g., frequency, percent, or words). The relationship between probability format and numeracy on comprehension of health risk probabilities was investigated via questions and delayed tests of recall for 36 younger adults' (Mage=20.0, SD=2.2, range=18-27) and 36 older adults' (Mage=71.1, SD=2.4, range=66-75). No interaction between numeracy and format was identified; higher numeracy was positively correlated with higher accuracy on comprehension questions across all formats. The results suggest that percent format best supports comprehension and recall of health risk probabilities for younger and older adults in a probability comparison task.