Illness-related malnutrition has been reported in 10-55% of people in hospital and in ill people in the community in areas of food sufficiency. It has been suggested that dietary counseling to encourage the use of energy- and protein-rich foods should be used in preference to oral nutritional supplements in the management of illness-related malnutrition. To examine the evidence that dietary advice, to improve nutritional intake, in adults with illness-related malnutrition can improve survival, weight and anthropometry and to estimate the size of any additional effect of whole protein nutritional supplements when given in combination with dietary advice. Six electronic databases were searched (Cochrane Library, Medline, EMBASE, CINAHL, ERIC and Dissertation Abstracts). Additional studies were sought by contacting dietitians, clinicians and the manufacturers of nutritional supplements. All randomised controlled trials of dietary advice compared with no advice, oral nutritional supplements and dietary advice plus oral nutritional supplements in people with illness-related malnutrition. Two reviewers independently assessed the trial eligibility, methodological quality and extracted the data. Fifteen trials (16 comparisons) met the inclusion criteria. An additional group of six trials were identified during searching which compared dietary advice plus supplements if required with no advice. These trials have been included in the review as an additional comparison. The review includes 1185 randomised participants from a variety of clinical backgrounds. The duration of follow-up was from six weeks to 12 months. Nine trials reported clearly concealed randomisation and allocation and two trials reported blinding of outcome assessment. Nine trials reported outcomes in an unusable format. Gain in weight was significantly higher for groups consuming supplements with or without advice, compared with advice alone. Energy intake was significantly improved in those consuming supplements compared with those receiving advice alone. There was insufficient data on group differences in functional outcomes and mortality. This review highlights the lack of evidence for the provision of dietary advice in the management of illness-related malnutrition. The available data suggest that oral nutritional supplements have a greater role than dietary advice in the improvement of body weight and energy intake. A large adequately powered randomised controlled trial is needed to compare the efficacy of different forms of therapy to increase dietary intake in people with illness-related malnutrition and to examine the impact of this on clinical function and survival.