Wear debris is produced by both the gross and microscopic motion between all implant, bone, and cement interfaces in prosthetic arthroplasty. It has been hypothesized that this debris elicits an inflammatory biological reaction, resulting in bone loss, implant loosening, and subsequent arthroplasty failure. It is generally accepted that factors influencing the bioreactivity of tissue to debris include the amount (mass burden) of debris and the size, shape, and surface properties of the particles. There have been several qualitative studies of particulates visible by light microscopy in human tissues, but there are few studies with clinically derived data on the submicrometre fraction. Therefore, in this study, data on the size and mass of submicrometre human synovial-derived arthroplasty wear debris have been obtained. The particulates extracted from both tissues and isolated macrophages recovered at revision surgery were relatively monodispersed, with a mean diameter of 425 nm, and constituted approximately 1.8% of the tissue mass. In a hypothetical example involving debris from a sample of ASTM F 75 alloy in a 6-g sample of synovium, these numbers translate to 2.9 × 1011 particles, with a combined surface area of 1600 cm2. These findings suggest that particulates smaller than 500 nm may have an effect on material biocompatibility in arthroplasty.