Although the prevalence of overweight and obesity in Australia has increased during the past 30 years, little is known about the dietary and behavioural antecedents of body mass index (BMI). We examined changes in mean BMI, diet, and other lifestyle behaviours between 1976 and 2005 and described the cross-sectional associations between these factors and BMI. A series of biennial biomedical surveys by Sydney Adventist Hospital from 1976 to 2005 allowed examination of BMI trends, while the selection of three surveys enabled detailed examination of likely dietary and lifestyle associations. Subjects included in this study were: 384 men and 338 women in 1976; 160 men and 146 women in 1978; 166 men and 141 women in 1980; 164 men and 142 women in 1982; 177 men and 13 women in 1984; 239 men and 227 women in 1986; 210 men and 225 women in 1988; 165 men and 148 women in 1990; 138 men and 167 women in 1992 and 270 men and 62 women in 2005. Height and weight were measured by hospital staff. Mean BMI increased in the early 1990s. Salt, coffee, cola, alcohol and meat consumption, dieting to lose weight and eating between meals were positively associated with BMI while physical activity, food variety, large breakfasts and consumption of spreads were negatively associated. Food consumption and daily activities have important associations with BMI, though their specific associations differ by sex. “Affluent” lifestyle patterns appear to contribute to higher BMI, while a more “prudent” lifestyle seems to protect from such increases.