Chronic malnutrition in children remains highly prevalent in Laos, particularly among ethnic minority groups. There is limited knowledge of specific nutrition practices among these groups. We explored nutritional status, cultural beliefs and practices of Laos' Khmu ethnic group to inform interventions for undernutrition as part of a Primary Health Care (PHC) project. Mixed methods were used. For background, we disaggregated anthropometric and behavioural indicators from Laos' Multiple Indicator Cluster Survey. We then conducted eight focus group discussions and 33 semi-structured interviews with Khmu villagers and health care workers, exploring beliefs and practices related to nutrition. The setting was two rural districts in Luang Prabang province, in one of which the PHC project had been established for 3 years. There was a higher prevalence of stunting in the Khmu than in other groups. Disaggregation showed nutrition behaviours were associated with ethnicity, including exclusive breastfeeding. Villagers described strong adherence to post-partum food restrictions for women, while little change was described in intake during pregnancy. Most children were breastfed, although early introduction of pre-lacteal foods was noted in the non-PHC district. There was widespread variation in introduction and diversity of complementary foods. Guidance came predominantly from the community, with some input from health care workers. Interventions to address undernutrition in Khmu communities should deliver clear, consistent messages on optimum nutrition behaviours. Emphasis should be placed on dietary diversity for pregnant and post-partum mothers, encouraging exclusive breastfeeding and timely, appropriate complementary feeding. The impact of wider governmental policies on food security needs to be further assessed.