Aminopeptidase N/CD13 is expressed by fibroblast-like synoviocytes (FLS) and monocytes (MNs) in inflamed human synovial tissue (ST). This study examined the role of soluble CD13 (sCD13) in angiogenesis, MN migration, phosphorylation of signaling molecules, and induction of arthritis. The contribution of sCD13 was examined in angiogenesis and MN migration using sCD13 and CD13-depleted rheumatoid arthritis (RA) synovial fluids (SFs). An enzymatically inactive mutant CD13 and intact wild-type (WT) CD13 were used to determine whether its enzymatic activity contributes to the arthritis-related functions. CD13-induced phosphorylation of signaling molecules was determined by Western blotting. The effect of sCD13 on cytokine secretion from RA ST and RA FLS was evaluated. sCD13 was injected into C57BL/6 mouse knees to assess its arthritogenicity. sCD13 induced angiogenesis and was a potent chemoattractant for MNs and U937 cells. Inhibitors of Erk1/2, Src, NF-κB, Jnk, and pertussis toxin, a G protein-coupled receptor inhibitor, decreased sCD13-stimulated chemotaxis. CD13-depleted RA SF induced significantly less MN migration than sham-depleted SF, and addition of mutant or WT CD13 to CD13-depleted RA SF equally restored MN migration. sCD13 and recombinant WT or mutant CD13 had similar effects on signaling molecule phosphorylation, indicating that the enzymatic activity of CD13 had no role in these functions. CD13 increased the expression of proinflammatory cytokines by RA FLS, and a CD13 neutralizing Ab inhibited cytokine secretion from RA ST organ culture. Mouse knee joints injected with CD13 exhibited increased circumference and proinflammatory mediator expression. These data support the concept that sCD13 plays a pivotal role in RA and acute inflammatory arthritis.