We describe an open, uncontrolled, observational study to evaluate the safety, acceptability and efficacy of a high-energy, fibre-containing oral nutritional supplement in patients who were either malnourished or who had experienced recent, acute weight loss and therefore needed a high-energy and/or fibre supplement.Assessments For assessment of safety and tolerance, occurrence of adverse events and oedema were to be documented during the 2-week trial and routine blood biochemistry was measured. The acceptability of the oral nutritional supplement was rated by the patients. In order to assess efficacy, patients’ body mass index (BMI) was calculated at baseline and following 2 weeks of nutritional intervention.Results A daily intake of 400–600 mL of a high-energy, fibre-containing liquid nutritional supplement was well accepted and no adverse events or problems with compliance were recorded. The high acceptability was underlined by favourable ratings of taste, smell, texture and overall acceptability. The patients’ median (interquartile range) BMI increased from 18 kg m–2 (18–19 kg m–2) to 20 kg m–2 (19–20 kg m–2); this increase was highly significant (P < 0.001).Conclusion We conclude that a high-energy, fibre-containing nutritional supplement is safe, well tolerated and associated with improved nutritional status in malnourished patients.