Objective: To examine the effect of Seventh-day Adventist (SDA) membership on ‘immunity’ to the secular effects of changes in BMI. Design: Three independent, cross-sectional, screening surveys conducted by Sydney Adventist Hospital in 1976, 1986 and 1988 and a survey conducted among residents of Melbourne in 2006. Subjects: Two hundred and fifty-two SDA and 464 non-SDA in 1976; 166 SDA and 291 non-SDA in 1986; 120 SDA and 300-non SDA in 1988; and 251 SDA and 294 non-SDA in 2006. Measurements: Height and weight measured by hospital staff in 1976, 1986 and 1988; self-reported by respondents in 2006. Results: The mean BMI of non-SDA men increased between 1986 and 2006 (P,0?001) but did not change for SDA men or non-SDA women. Despite small increases in SDA women’s mean BMI (P50?030) between 1988 and 2006, this was no different to that of SDA men and non-SDA women in 2006. The diet and eating patterns of SDA men and women were more ‘prudent’ than those of non- SDA men and women, including more fruit, vegetables, grains, nuts and legumes, and less alcohol, meat, sweetened drinks and coffee. Many of these factors were found to be predictors of lower BMI. Conclusion: The ‘prudent’ dietary and lifestyle prescriptions of SDA men appear to have ‘immunised’ them to the secular effects of changes that occurred among non-SDA men’s BMI. The dietary and lifestyle trends of SDA women did not reflect the increase in their BMI observed in 2006.