OBJECTIVES--To investigate the hypothesis that synovial fluid (SF) from patients with osteoarthritis (OA) may contain calcium phosphate crystals that are either too small, or too few in number to be identified by conventional light microscopy techniques. METHODS--Twelve SF from 11 patients with established knee OA, five SF from patients with rheumatoid arthritis (RA), and two control samples of SF from patients with pseudogout were subjected to an enzyme/hypochlorite extraction procedure. The patients with OA and RA had no radiographic evidence of chondrocalcinosis, or SF crystals on polarised light microscopy. Extracted material was examined and analysed by analytical electron microscopy (AEM) and x ray powder diffraction (XRD). RESULTS--Mineral was found in 11 of 12 OA samples, ranging from 2-120 micrograms/ml SF. Analytical electron microscopy revealed calcium pyrophosphate dihydrate (CPPD) crystals in five (confirmed by XRD in three) and basic calcium phosphates (BCP) in eight (five on XRD). Two samples with confirmed CPPD contained some rods with a mean length below 100 nm. The majority of BCP clusters were also less than 100 nm in diameter. BCP was detected in 1/5 RA samples. Control samples contained CPPD crystals of the expected size range of 0.42-17.9 microns. CONCLUSIONS--The data indicate that many OA SF may contain CPPD or BCP crystals which are too small or too few in number to be identified by conventional techniques. Crystal deposition is not an 'on-off' phenomenon in OA.