The association between calcium pyrophosphate dihydrate (CPPD) crystal deposition and ageing is unexplained. Levels of inorganic pyrophosphate (PPi) and activity of nucleoside triphosphate pyrophosphatase (NTPP) were estimated in synovial fluid from 91 asymptomatic normal knees, including five with isolated CPPD crystal deposition (70 subjects; median age 47, range 22-83 years). In non-crystal fluids, PPi levels and NTPP activities were low (median, interquartile range: 9.5, 7.6-12.3 microM, and 1, 0.4-2 microM/30 min/mg protein, respectively: n = 50) and a generalized increase with age was not demonstrated. Higher values for PPi and NTPP in all five subjects with isolated CPPD deposition supports involvement of altered PPi metabolism, unrelated to ageing per se, in crystal formation. Presumed cartilage fragments were identified by morphological characteristics in 70% of normal fluids: metachromatic staining and electron microscopy, undertaken in a limited number, supported identification of such particles as cartilage. This finding supports dynamic turnover of healthy matrix, and questions the usefulness of identification, without quantification, cartilage fragments as a measure of cartilage damage.