BACKGROUND: Integrative and complementary health approaches (ICHA) are often pursued by patients facing chronic illnesses. Most of the studies that investigated the factors associated with ICHA consumption have considered that the propensity to use ICHA is a stable or fixed characteristic of an individual. However, people may prefer using ICHA in some situations and not in others, depending on the characteristics of the illness to face. Moreover, the attitude toward ICHA may differ within a single individual and between individuals so that ICHA can be used either in addition to (i.e., complementary attitude) or in place of (i.e., alternative attitude). The present study aimed at examining distinct patterns of attitudes toward ICHA in people hypothetically facing chronic illnesses that differed according to severity and clinical expression. METHODS: We conducted a web-based study including 1807 participants who were asked to imagine that they had a particular chronic illness based on clinical vignettes (mental illnesses: depression, schizophrenia; somatic illnesses: rheumatoid arthritis, multiple sclerosis). Participants were invited to rate their perceived distress and social stigma associated with each illness as well as its perceived treatability. They also rated their belief in treatment effectiveness, and their treatment preference. Four patterns of treatment choice were determined: strictly conventional, weak or strong complementary, and alternative. Bayesian methods were used for statistical analyses. RESULTS: ICHA were selected as complementary treatment option by more than 95% of people who hypothetically faced chronic illness. The complementary attitude towards ICHA (in addition to conventional treatment) was more frequent than the alternative one (in place of conventional treatment). Factors driving this preference included employment status, severity of illness, age and perceived distress, social stigma and treatability of the illness. When the label of illnesses was included in the vignettes, patterns of treatment preference were altered. CONCLUSIONS: This study provides evidence that "medical pluralism" (i.e., the integration of ICHA with conventional treatment) is likely the norm for people facing both mental or somatic illness. However, our result must be interpreted with caution due to the virtual nature of this study. We suggest that taking attitudes toward ICHA into account is crucial for a better understanding of patients' motivation to use ICHA.