This paper investigates the structural reliability of a discrete choice experiment within health technology assessment. Two versions of a discrete choice experiment, in the form of a self-completion questionnaire, were randomly administered to two samples of women who had recently given birth as part of an exercise to determine women's preferences for alternative modes of intrapartum care. In the first questionnaire, two of the attributes had only their highest and lowest levels included, while in the second questionnaire all three levels for these two attributes were included. The levels included for all other attributes remained the same throughout both questionnaires. The evidence relating to the structural reliability of the discrete choice experiment in this context was mixed. The results indicated that the relative importance of the two attributes in which the levels were varied increased as the number of levels for these attributes increased. However, the relative importance of the attributes in which the levels were not varied remained relatively stable throughout. The results provide evidence in support of a psychological effect whereby respondents place more importance upon specific attributes as the number of levels for these attributes increases. It is recommended that further research of both a qualitative and quantitative nature should be undertaken to assess the potential importance (or otherwise) of a psychological effect relating to the number of levels chosen for attributes within discrete choice experiments in health technology assessment.