Fortifying complementary foods with lipid-based nutrient supplements (LNS) may improve energy and nutrient intakes of infants at risk for undernutrition. We aimed to determine the relative validity of an interactive 24-h recall (i-24-HR) for assessing the impact of an LNS intervention on dietary intakes of energy and nutrients among rural Malawian 9–10-month-old infants (n 132) participating in the International Lipid‐Based Nutrient Supplements Dose (iLiNS-DOSE) trial. Dietary data were collected for the same day via i-24-HR and weighed food records. Inter-method agreements were estimated overall and by intervention group, using Bland–Altman plots and paired t tests; measurement error models (differential error); and percentage of food omissions and intrusions were estimated. Overall, inter-method differences in mean intakes of energy and most nutrients were not significant. When stratified by group, recalled energy intakes were under-estimated (−368 kJ; P=0·01) in the control but not in the intervention group (−42 kJ; P=0·6). This differential reporting error was related to an over-estimation of recalled LNS (8·1 v. 4·5 g; P<0·001) in the intervention group, compensating for an under-estimation of energy and nutrient intakes from complementary foods. Sources of measurement error in the i-24-HR were under-estimations of starchy staples, meat/fish/eggs and legumes/nuts/seeds (overall percentage agreement between 38 and 89 %; P<0·028); and over-estimations of added sugar, soups/broths and LNS (overall percentage agreement between 138 and 149 %; P<0·001). Common (>30 % eating occasions) omissions were milk/fish/eggs, starchy roots/vegetables and sweetened snacks. Common intrusions were milk/yogurt. Starchy staples and LNS were recalled when consumed (>85 %) (i.e. matched). These results emphasise the importance of considering differential error when interpreting dietary results in LNS trials.