Six men who had undergone hip replacements for degenerative joint disease or trauma subsequently had radical prostatectomies or cystoprostatectomies with bilateral pelvic lymph node dissections for adenocarcinoma of the prostate or transitional cell carcinoma of the urinary bladder. The hip prostheses implanted in three patients were known to contain cobalt-chromium alloy and titanium. The pelvic lymph nodes ipsilateral to the hip prosthesis in five patients and the bilateral pelvic nodes in the only patient with bilateral hip prosthesis had dark brown or black cut surfaces. These lymph nodes did not contain carcinoma but showed florid sinus histiocytosis characterized by large polygonal histiocytes filling and expanding sinuses and interfollicular regions. The foamy histiocytes contained cobalt-chromium and titanium micropar-ticles by light microscopy, ultrastructure, and energy-dispersive x-ray microanalysis. The lymph nodes uninvolved by the histiocytic reaction lacked the heavy metal microparticles. Four cases were found to have a small number of polyethylene particles, which might have contributed to the histiocytic response. By immunohis-tochemistry, the foamy cells displayed immunoreactivity for lysozyme, α-1-antitrypsin, α-1-antichymotrypsin, and cathepsin D, providing additional support for their histiocytic derivation. To our knowledge, this is the first time that microparticles of cobalt-chromium and titanium that migrate from hip prostheses to pelvic lymph nodes have been shown to elicit a distinctive type of florid sinus histiocytosis. Pathologists should be aware of this characteristic foreign-body tissue response to avoid confusion with other types of sinus histiocytosis or with metastatic carcinoma.