Photodynamic therapy (PDT) is a promising treatment modality that has recently been accepted in clinics as a curative or palliative therapy for cancer and other nonmalignant conditions. Phthalocyanines (Pc) are attractive photosensitizers for PDT because of their enhanced photophysical and photochemical properties. The overall charge and solubility of Pc play a major role in their potential usefulness for PDT. A series of amphiphilic derivatives of tetrasulfonated aluminum Pc (AlPcS₄) was prepared by substituting one of the four sulfonate groups with aliphatic side chains of 4, 8, 12 and 16 carbon atoms. The photodynamic properties of the derivatives were compared with those of AlPcS₄ and the adjacent disulfonated aluminum Pc. Parameters studied included reversed-phase high-performance liquid chromatography (HPLC) retention times, capacity to generate singlet oxygen (¹O₂), in vitro cell uptake and phototoxicity, as well as PDT response of transplantable EMT-6 tumors in mice. The monomerized AlPcS₄ derivatives showed similar or higher capacities to generate ¹O₂ as compared with the parent AlPcS₄ as measured from relative l-tryptophan photooxidation yields. A549 cell uptake of the AlPcS₄ derivatives decreased in the following order: AlPcS₄(C16) > AlPcS₄(C12) > AlPcS₄(C8) > AlPcS₄(C4). Human low-density lipoprotein at high concentrations (40 μg/mL) completely prevented uptake, whereas at 4 μg/mL uptake was decreased for the more lipophilic compounds and yet remained unaffected for the more hydrophilic dyes. Using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, A549 cell survival was assessed; it showed that photocytotoxic activity varied directly with the HPLC retention times, i.e. more hydrophilic compounds were less phototoxic. As ¹O₂ yields were similar for the four substituted AlPcS₄ derivatives, it was postulated that the increased cytotoxic activity was caused by enhanced subcellular localization as a result of the long aliphatic side chains. These amphiphilic compounds proved to be photodynamically potent against the EMT-6 mouse mammary tumor model implanted in Balb/c mice. At dye doses of 0.2 μmol/kg and a fluence of 400 J/cm² complete tumor regression was observed with no morbidity. The substitution of AlPcS₄ with long aliphatic chains on the macrocycle greatly enhances its photodynamic efficacy both in vitro and in vivo.