Frank cachexia is usually evident in the clinical evaluation, but important metabolic alterations in tumour-bearing patients with slight weight changes are frequently overlooked. Since several factors are involved in cancer-induced malnutrition (marasmus type descriptive of general inadequate food intake, kwashiorkor type descriptive of inadequate protein intake/increased turnover), medical/dietary history and physical examination (dynamic weight loss, vitamin/mineral deficiency) should be complemented by simple additional laboratory tests. Minimal baseline testing may include: weight loss (> 5%/month = severe) and serum albumin (< 24 g/l = severe). Anthropometric measurements (triceps skin fold, midarm muscle circumference) for the assessment of fat deposits and lean body mass are rarely used in a routine clinical setting owing to great variations among individuals and interobserver measurement variability. The Prognostic Nutritional Index (PNI), including serum parameters, immune competence testing and anthropometrics (but no further dietary parameters, i.e. dynamic weight loss), has proved to be quite a reliable method of predicting postoperative morbidity/mortality and of selecting cancer patients for aggressive preoperative nutritional interventions. For clinical studies including the evaluation of nutritional status (stratification, prognostic impact) and/or outcome (nutritional interventions), a simple standardized and validated assessment protocol including dietary history, specific physical examination and widely available laboratory testing should be implemented to document significant nutritional alterations. Body weight alone will often fail to demonstrate important disease- or therapy-related changes in caloric intake or metabolic rate.