We developed the Symptoms of Illness Checklist (SIC) to study psychological influences on physical symptoms of illness. A total of 520 participants completed the SIC and, in some samples, the Salient Stressor Impact Questionnaire, Perceived Stress Scale, Daily Hassles and Uplifts Scale, Derogatis Stress Profile, Life Experiences Survey, and the Symptoms CheckList-90-R. The SIC's test-retest, internal reliability, and validity verified by physician ratings, were very good. SIC correlations with diverse stress measures were less inflated than those of other instruments indicating the SIC is a reliable and valid tool to study psychological influences on physical illness.