The Changes in Sexual Functioning Questionnaire (CSFQ), a structured interview/questionnaire designed to measure illness- and medication-related changes in sexual functioning, is presented with evidence of its validity and reliability. Medical students (n = 122) and psychiatry residents (n = 33) completed the CSFQ on two separate occasions. Residents also completed the Derogatis Interview for Sexual Functioning-Self-Report (DISF-SR), a reliable, validated measure used in research on sexual functioning in patient populations. Concurrent validity was established between the CSFQ and the DISF-SR and test-retest reliability was high. The CSFQ is a reliable and valid measure of sexual functioning, useful in both clinical and research settings.