For molecular weights up to 150, all molecular graphs corresponding to possible organic compounds made of C, H, N, O were generated using the structure generator MOLGEN. The numbers obtained were compared to the numbers of molecular graphs corresponding to actually known compounds as retrieved from the Beilstein file. The results suggest that the overwhelming majority of all organic compounds (even in this low molecular weight range) is unknown. Within the set of C ₆H ₆ isomers, a very crude and a highly sophisticated energy content calculation perform amazingly similar in predicting a particular structure's existence as a known compound.