Research has shown that patients with somatoform disorders (SFD) have difficulty using medical reassurance (i.e. normal results from diagnostic testing) to revise concerns about being seriously ill. In this brief report, we investigated whether deficits in adequately interpreting the likelihood of a medical disease may contribute to this difficulty, and whether patients’ concerns are altered by different likelihood framings.

Patients with SFD (N = 60), patients with major depression (N = 32), and healthy volunteers (N = 37) were presented with varying likelihoods for the presence of a serious medical disease and were asked how concerned they are about it. The likelihood itself was varied, as was the format in which it was presented (i.e. negative framing focusing on the presence of a disease v. positive framing emphasizing its absence; use of natural frequencies v. percentages).

Patients with SFD reported significantly more concern than depressed patients and healthy people in response to low likelihoods (i.e. 1: 100 000 to 1:10), while the groups were similarly concerned for likelihoods ⩾1:5. Across samples, the same mathematical likelihood caused significantly different levels of concern depending on how it was framed, with the lowest degree of concern for a positive framing approach and higher concern for natural frequencies (e.g. 1:100) than for percentages (e.g. 1%).

The results suggest a specific deficit of patients with SFD in interpreting low likelihoods for the presence of a medical disease. Positive framing approaches and the use of percentages rather than natural frequencies can lower the degree of concern.