OBJECTIVE: To determine the methodology to be used in a national survey on sexual behaviour in France.¦DESIGN: To compare two modes of investigation: telephone and face-to-face surveys.¦METHODS: A questionnaire lasting between 15 and 60 min was administered to 800 people, one-half of whom were interviewed by telephone and the other half face-to-face.¦RESULTS: We found that questions were more easily answered and answers more coherent in the telephone survey. A face-to-face survey, in contrast, generates a greater diversity in answers and reduces the tendency to give answers relating to behaviours or attitudes that are socially desirable. Results also showed that the acceptation rates were similar in both surveys (66 versus 67%).¦CONCLUSIONS: Each method has its own advantages and disadvantages; it is therefore difficult to choose one survey method over the other.