The evaluation of cancer pain remains a problematic clinical problem, not only due to the subjective and multidimensional nature of pain per se, but also because of its specific characteristics. Cancer pain has an insidious onset, often involves many sites, and is frequently multicausal. Tools have been developed to quantify pain, the most commonly used being the verbal rating scale (VRS), the visual analogue scale (VAS), and the McGill Pain Questionnaire (MPO). The first 2 scales are short, easy to administer and to score, but only measure pain intensity. The VRS is assumed to be an ordinal scale although unequal differences between pain descriptors have been demonstrated; it offers a restrictive choice of words that may not represent pain experience with sufficient precision, and is not sensitive to change especially for mild pain. The VAS on the other hand, represents pain as a continuum and is sensitive to change. The MPQ has the advantage of evaluating the sensory, affective and evaluative dimensions of pain. However, it is lengthy to administer and some words are not readily understandable. In addition, the words within a given category are considered to be equidistant, the number of words in each category are unequal, and the number of categories evaluating a given dimension are not taken into account when calculating the total pain rating index. A further issue in assessing pain, other than the choice of a valid and reliable tool, is the frequency with which pain evaluations should be repeated. To date no studies have addressed this problem.(ABSTRACT TRUNCATED AT 250 WORDS)