This paper reports on the development, reliability, and validity of a new self-report instrument: the Sexual Interaction System Scale (SISS). Initial research on 143 couples supports reliability as measured by the internal consistency of the five factors. Validity is supported by several tests: the SISS differentiated between a nonclinical group and couples in therapy for sex dysfunction and also between two clinical groups--couples with sex dysfunction and couples with other problems. It also correlated strongly with a measure of sexual satisfaction and with the Affection subscale and overall score of the Dyadic Adjustment Scale. Discussion focuses on interpretation of results via case examples and on the utility of the SISS in research and clinical practice.