In the course of two studies investigating whether individuals with recurrent tension headache differ from headache-free controls in the frequency and appraisal of stressful life events, subjects were asked to complete a headache diary for I week. Subjects in the headache-free control group were screened twice and reported having infrequent headaches (two or less per month). However, 13 of 38 control subjects reported multiple headache days on their headache diaries. These subjects also reported a greater frequency of stressful events and a greater stress impact score. These results suggest that a careful screening of control subjects may be necessary in studies theft examine differences between subjects with recurrent headache and headache-free controls. There is also some evidence that the inclusion of these subjects in a control group may bias the sample, making it less likely to find between-group differences.