Household composition influences people's diet, so typical transitions in young women's lives, including cohabitation, marriage, and motherhood, might be expected to influence their subsequent dietary behavior. The objective was to examine associations between transitions in living arrangements and changes in energy intake and dietary patterns for women in their 20s and 30s using longitudinal data collected in 2003 and 2009. FFQ were collected twice from 6534 women born in 1973-1978 participating in the Australian Longitudinal Study on Women's Health. Transition groups were defined from changes in their living arrangements. Factor analysis was used to identify dietary patterns. Associations between transitions in living arrangements and changes in energy intake and dietary pattern scores were analyzed using multiple linear regression. Women living with children had greater energy intake than other women initially and those who started a family had the greatest increases over time. Five similar dietary patterns were derived from both surveys. Women living in a family at both times had higher scores on the high-fat and sugar, meat, and cooked vegetables patterns and lower scores on the Mediterranean-style and fruit patterns than other women. Women starting a family increased their consumption of the high-fat and sugar, fruit, and cooked vegetables patterns. Women not living with children at both times had increased scores on the Mediterranean-style pattern and decreased scores on the high-fat and sugar and cooked vegetables patterns compared with other women. In conclusion, starting a family is associated with changes in women's diet that are mainly unhealthy.