In scientific papers, skeletal maturation-expressed as bone maturity scores or bone age-is often used as a quantifiable variable similar to height or weight. This paper discusses whether this approach is appropriate. The questions addressed are whether skeletal maturation can be measured on a quantitative scale, whether its use is appropriate in computing, and what the 'numbers' used represent. Reference will be made mainly to the Tanner-Whitehouse method, which, in the opinion of the authors, has been the most reliable method of assessment to date. Many of the remarks made in this paper may be extended to other methods of assessment, and have been stressed by Tanner himself. The authors are aware that, in the future, some of the remarks could be made redundant by the development of more detailed definitions of bone maturation. This is becoming feasible with the advent of expert systems for the automatic recognition of different stages of bone maturation.