This paper studies the comparative statics of adding agents to matching markets that generalize the marriage and college-admissions markets of D. Gale and L. Shapley (Amer. Math. Monthly 69, 1962, 9-15). It is shown, for the wide class of matching markets studied by A. Roth (Econometrica 52, 1984, 47-57), that adding an agent to one side of the market weakens the competitive positions of the other agents on that side and strengthens the competitive positions of the agents on the other side. © 1991.