Configurational-bias Monte Carlo simulations in the isobaric-isothermal ensemble using the nonpolarizable TraPPE-UA (transferable potentials for phase equilibria-united-atom) force field were performed to study the aggregation of l-hexanol in n-hexane. The spatial distribution of alcohols was sampled efficiently using special Monte Carlo moves. Analysis of the microscopic structures for 1, 3, and 5% solutions at a temperature of 298.15 K and a pressure of 101.3 kPa shows strong aggregation with a preference for tetramers and pentamers for all three concentrations. About half of these tetramers and pentamers are found in cyclic aggregates. The enthalpies for the formation of clusters of specific sizes were determined from simulations of a 3% solution at temperatures ranging from 298.15 to 328.15 K. The free energies and enthalpies of cluster formation show the large influences of hydrogen-bond cooperativity, which favors clusters larger than dimers, but a decreasing enthalpy gain and an increasing entropic penalty prevent the formation of very large clusters. These results have important implications for the thermodynamic modeling of hydrogen-bonding fluids, which commonly use a constant value for the free energy of hydrogen-bond formation. Overall agreement with Fourier transform infrared spectroscopic measurements on the extent of hydrogen bonding for the same mixtures is satisfactory.