IntroductionGrowth shortfalls and diarrhoeal diseases remain a major cause of morbidity and mortality in low-income settings. Due to the multifaceted causes of undernutrition and the identified limitations of siloed nutrition programmes, improving the delivery of integrated water, sanitation, hygiene (WASH) and nutrition programming could improve child health.MethodsWe conducted a cluster randomised trial in western Kenya to assess the impact on household behaviours of a novel, theory-informed and integrated WASH and nutrition intervention delivered through care groups as compared with the standard care group approach. We developed an intervention targeting practices relating to food hygiene, mealtime and feeding, and compound cleanliness, each using various behavioural change techniques to influence the uptake of targeted behaviours. Prespecified behavioural outcomes were verified through direct observation, 24āhours recall, and self-reported picture-based methods.ResultsCompared with control households, a greater proportion of intervention households had a hygienic food preparation area (Risk double difference (RDD) 0.81, 95%āCI 0.68 to 0.96), had stored food hygienically (RDD 0.76, 95%āCI 0.58 to 1.00), had a functional handwashing station (RDD 0.64, 95%āCI 0.56 to 0.74), provided a safe space for their child to play (RDD 0.73, 95%āCI 0.56 to 0.96), and who fed their children thickened porridge (RDD 0.56, 95%āCI 0.51 to 0.63) at endline. The proportion of children 6ā24 months in intervention households consuming a sufficient diversity of foods (RDD 0.81, 95%āCI 0.64 to 1.04) was higher than in control households; however, there was a non-significant increase in the percentage of pregnant and lactating women receiving an adequate diversity of foods in their diets (RDD 0.86, 95%āCI 0.70 to 1.05) among intervention compared with control households at endline.ConclusionOur integrated WASH and nutrition intervention resulted in important changes in behaviours. This theory-informed intervention could be added to existing care group programmes to considerable advantage.