Many studies have shown that people have difficulty judging the diagnostic value of conditional probability information with respect to one or more hypotheses. The present research addressed two aspects of performing the diagnostic task in a health care decision: (a) recognition of the information's importance, and (b) correct usage of that information. In experiment 1, health care providers, who are trained in, and regularly exposed, to conditional probabilities imparting diagnostic information, exhibited at least a rudimentary recognition of the need for this information in assessing diagnosticity. Experiment 2 indicated that health care and layperson subjects had difficulty in actually applying the information, however. This difficulty prompts a need for judgment aids and caution in using diagnostic information.