The traditional end-to-end cyclization of long-chain linear precursors is difficult and often unpredictable because the unfavorable entropy of macrocyclic closure allows undesired intermolecular reactions to compete. Here, we apply cavitands to the selective intramolecular aldol/dehydration reaction of long-chain alpha,omega-dialdehydes in aqueous solution. Hydrophobic forces drive the dialdehydes into the cavitands in folded conformations and favor macrocyclization reactions over intermolecular reactions observed in bulk solution. The macrocyclic aldol reaction products are isolated in good yields (30-85%) over a wide range (11 to 17-membered rings). Unlike conventional templates that become guests inside their assembled hosts, cavitands reverse the roles and resemble the situation in biological catalysis-the templates are hosts for guests undergoing the assisted reaction processes.