To test the hypothesis that a micronutrient supplement can improve seroconversion after influenza immunisation in older institutionalised people, a randomised, double-blind, placebo-controlled study was initiated. A total of 164 residents aged 60 and older from 31 nursing and residential homes were initially randomised; of these, 119 (72.6%) completed the study. Participants were randomised to receive a micronutrient supplement providing the reference nutrient intake for all vitamins and trace elements or identical placebo. Tablets were taken over an 8-week period during September and October 2000; influenza vaccine was administered 4 weeks after their commencement. The haemagglutination-inhibiting antibody response was defined by a fourfold or greater titre rise over 4 weeks and assessed separately for each of the three antigens contained in the 2000/2001 influenza vaccine [A/New Caledonia/20/99 (H1N1), A/Moscow/10/99 (H3N2), B/Beijing/184/93 (B)]. Despite a significant increase in serum concentrations of vitamins A, C, D3, E, folate and selenium in the supplemented group, there was no significant difference between groups in the proportion of participants seroconverting to H1N1 (41 vs. 49%, P = 0.374), H3N2 (49 vs. 58%, P = 0.343) or B (41 vs. 40%, P = 0.944).