Mycobacterium fortuitum is classified as a rapidly growing mycobacterium (R
GM) according to the Runyon classification. RGM are increasingly being reco
gnized as human pathogens. Joint infection due to M. fortuitum is a rare, b
ut serious disease. This report describes a patient with acquired immunodef
iciency syndrome (AIDS) and septic arthritis of the knee due to M. fortuitu
m in a previously normal joint with no history of surgery or intra-articula
r injections.