Background The aim of the study was to evaluate the frequency of cholesterol crystals occurrence in synovial fluid (SF) in rheumatic diseases as well as characteristics of crystals.

Objectives We have studied 1000 synovial fluid samples from 854 patients. The most numerous was group of rheumatoid arthritis (RA) patients (573 cases) and osteoarthritis (OA).

Methods Synovial fluids were taken on heparin as anticoagulant. SF sediments were examined under polarised microscope.

Results Cholesterol crystals were demonstrated in 16 samples taken from 7 patients with RA only. Most of the crystals occurred as typical large flat rectangular or romboid plates. They ranged in size from 8 to 100 um and they showed strong birefringence with negative or positive elongation. Less often, they appeared as long rods or curved needle-shaped crystals.

Conclusion The results showed rare occurrence of cholesterol crystals in synovial fluid samples. Polymorphism of cholesterol crystals under polarised microscope may be a cause of difficulties in synovial crystals differentiation.