A mixture of the RR/SS and RS/SR diastereoisomeric pairs of methyl 4-(2,4-dichlorophenyl)-2,7-dimethyl-5-oxo- 1,4,5,6,7,8-hexahydroquinoline-3-carboxylate, C19H19Cl2NO3, forms cocrystals in which there is one unique molecule in the asymmetric unit, but the molecule displays disorder in the region of the 7-position of the quinoline ring system as a result of the random occurrence of the diastereoisomers at the same crystallographic site. A similar arrangement exists in the monohydrate cocrystals that form from a mixture of the RR/ SS and RS/SR diastereoisomeric pairs of methyl 4-( 2,4-dichlorophenyl)- 2-methyl-7-phenyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3- carboxylate monohydrate, C24H21Cl2NO3 center dot-H2O. These compounds belong to a class of 1,4-dihydropyridines whose members have calcium modulatory properties. The 1,4- dihydropyridine rings have the usual shallow boat conformation. In each structure, the 2,4- dichlorophenyl ring is oriented such that the 2-chloro substituent is in a synperiplanar orientation with respect to the 1,4- dihydropyridine ring plane. In each crystal structure, the molecules are linked into chains by N- H center dot center dot center dot O hydrogen-bonding interactions.