We report a case of septic arthritis caused by the fastidious gram-negative rod Campylobacter fetus. We suggest that the organism may be part of the endogenous flora and that the clinical infections tend to occur in compromised hosts. Our patient is the first to be described with multiple myeloma and C. fetus septic arthritis. The documented cases of culture-proven C. fetus septic arthritis reported to date have occurred in three men and one woman, all in the seventh and eighth decades of life, with a mono-articular large joint distribution. The septic arthritis always occurred in previously injured joints and curiously enough need not be associated with a toxic-appearing patient. C. fetus infections are also associated with the signs and symptoms of clinical thrombophlebitis. We stress caution in establishing this diagnosis of phlebitis on clinical evaluation only and urge differentiation of true deep vein thrombophlebitis from pseudothrombophlebitis or dissected popliteal synovial cyst. This latter diagnosis may be made non-invasively by ultrasound techniques.