Subcutaneous fat biopsy was investigated for its sensitivity in giving a diagnosis in 44 consecutive patients with rheumatoid arthritis or ankylosing spondylitis suspected of systemic amyloidosis. In 26 of these patients amyloidosis could be demonstrated by fat or rectal biopsy or biopsies from organs suspected of amyloid deposition. Fourteen of the 26 (54%) fat biopsy specimens of the patients with amyloidosis were positive after staining with Congo red and 22 (85%) of the rectal biopsy specimens were positive. All 12 kidney biopsy specimens and 4 biopsy specimens from other organs of these 26 patients were positive for amyloidosis. In 2 patients with a negative rectal biopsy specimen, fat biopsy would have obviated the need for a more invasive biopsy. All patients experienced fat biopsy as less demanding compared to other biopsy procedures. These results imply that in patients with chronic arthritis subcutaneous fat biopsy is a useful screening procedure. In this patient group fat biopsy is less sensitive for the diagnosis of amyloidosis compared to rectal biopsy.