One-dimensional hydrogen-bonding aggregates of urea and thiourea corresponding to the two patterns, chains and ribbons, which are found in crystal structures of these molecules have been studied using ab initio and semiempirical molecular orbital theory. In accord with experimental evidence, long chains are found to be more stable than comparably sized ribbons for urea, although ribbons are favored for dimers and small aggregates. Thiourea ribbons are favored over chains for all aggregates, large and small, again in agreement with the experimental crystal structure. Thus, cooperative interactions dictate the hydrogen-bonding structure for urea, but not thiourea, crystals. Various ab initio (HF, MP2, and B3PW91 all the with the D95** basis set) and semiempirical (AM1 and SAM1) molecular orbital calculations were used for this study.