This experiment examines one component of worry, elevated subjective probabilities of negative events, and attempts to elucidate the cognitive processes on which this is based. The results suggest that the pessimistic subjective probabilities shown by chronic worriers can be understood using general theories of judgment, specifically, by the use of the availability heuristic (Tversky & Kahneman, 1973). However, it is the availability of a particular pattern of cognitions--an increased accessibility of explanations for why a negative event would occur, combined with a reduced accessibility of explanations for why it would not--that is important. The results are integrated within a description of the worry process, and possible clinical applications through the use of reason-generation techniques are discussed.