Bis(tert-butyldimethylsiloxy)- (7), bis(dimethylthexylsiloxy)- (8), bis(tri-n-hexylsiloxy)- (9), and bis(dimethyloctadecylsiloxy)silicon 2,3-naphthalocyanines (10) were prepared via substitution of the bis(hydroxy) precursor with the corresponding chlorosilane ligands and characterized by spectroscopic and combustion analyses. They show strong absorption around 780 nm where tissues exhibit optimal transparency. Compounds 7-10 are capable of producing singlet oxygen. They are relatively photostable although less stable than the analogous phthalocyanine, i.e., the bis-(dimethylthexylsiloxy)silicon phthalocyanine (12). They were evaluated as potential photosensitizers for the photodynamic therapy (PDT) of cancer in vitro against V-79 cells and in vivo against the EMT-6 tumor in Balb/c mice. In vitro all four dyes showed limited phototoxicity combined with substantial dark toxicity. Surprisingly, in vivo (i.v., 0.1 mumol/kg, 24 h prior to the photoirradiation of the tumor with 780-nm light, 190 mW/cm2, 400 J/cm2) all dyes induced tumor regression in at least 50% of mice whereas compound 8 gave a complete tumor response in 80% of mice without apparent systemic toxicity at doses as high as 10 mumol/kg. At 24 h postinjection, compound 8 showed a favorable tumor to muscle ratio of 7, assuring minimal damage to the healthy tissue surrounding the tumor during PDT. Our data confirm the potential of silicon naphthalocyanines as far-red-shifted photosensitizers for the PDT of cancer and indicate the importance of the selection of the two axial silicon ligands for optimal photodynamic efficacy.