Pore formation by electrochemical etching of p-type silicon in organic electrolytes has been studied. (100)-oriented wafers of silicon were prepatterned using a standard lithographic process to form pits of a known spatial period that act as nucleation sites for the pore formation. It was found for the first time that, for the prepatterned samples, two types of macro-pores could be formed on the same substrate. The first type of pore originates from the pits obtained by the prepatterning process. The second type is formed spontaneously as a result of a self-organization process. These ‘additional’ pores appear if the period of the printed pit pattern is higher than the intrinsic spatial period of random pores for Si, of a certain resistivity. The two types of macro-pores manifest different morphologies. Non-patterned samples oriented in (100), (111) or (110) plane were also investigated.