OBJECTIVE Sexual dysfunction (SD) severely affects the quality of life in patients with multiple sclerosis (MS). The aim of this study is to investigate the type and frequency of sexual complaints in MS patients, to analyse their relationship to various clinical and psychosocial variables and to clarify the differences between MS patients with and without SD. METHODS Thirty-five relapsing-remitting (RR), nine secondary progressive and seven primary progressive MS patients were included in this study. A structured face-to-face interview regarding sexual function and other physical problems which may interfere with sexual functioning was administered to each patient. They also filled out Multiple Sclerosis Intimacy and Sexuality Questionnaire-19 (MSISQ-19), which includes items for primary (direct physical), secondary (indirect physical) and tertiary (psychosocial) causes of SD. Disability, cognitive functions and psychological functioning were also evaluated. RESULTS Forty-one patients (80.4%) reported primary SD; decreased libido was the most frequent complaint (80.5%). These patients were older and more disabled, however 39% had low disability scores. SD was a common problem for both men and women. Patients with RRMS were affected less in all items of primary SD. Several items of secondary SD--problems with memory and concentration, bladder symptoms, bowel symptoms--showed correlation with different items of primary SD; these were altered genital sensation, decreased libido, increased time for arousal, decreased lubrication/difficulties with erection. Total MSISQ-19 scores were correlated with disease duration, age, disability, disease course, Beck depression scale, temporary and long-standing anxiety and low level of education. CONCLUSIONS SD is an underestimated, common symptom of MS. It may occur in MS even in the absence of severe disability. Physicians' awareness of this problem may help to bring about appropriate treatments and management, and improve the quality of life for these patients.