A report by M.A. Smith and P. Tothill (see ibid., vol.27, p.1515-21, 1982) is commented on. That report suggested, without evidence, that intra-laboratory variation in measurements of the bone mineral of the spine may be due to a technical problem termed 'crossover'. The authors feel that Smith and Tothill are to be commended for drawing attention to the matter of crossover, but also that they have misrepresented the procedures of others and have alluded to intra-laboratory differences where none have been shown to exist.