Iontophoretic transport of the prodrug 5-aminolevulinic acid (ALA), which is used for photodynamic therapy (PDT), across human stratum corneum (SC) was studied quantitatively in vitro. The experiments were carried out in a three-compartment iontophoresis cell consisting of two electrode chambers equipped with Ag–AgCl electrodes, each separated from a central acceptor chamber by a sheet of SC, supported by a dialysis membrane, to mimic the side-by-side configuration normally used in vivo. Acceptor fluid samples were collected every hour for a period of 30 h in a fraction collector and analyzed by high-performance liquid chromatography–fluorometry after derivatization of the ALA. The iontophoretic ALA flux was studied as a function of the applied current density and the ALA concentration in the donor solution (1, 2.5 or 10% ALA). Depending on the ALA concentration in the donor cell, iontophoresis enhances the flux from close to the detection limit of 0.23 nmol cm⁻² h⁻¹ at zero current density (passive diffusion) to several hundred or thousand nanomoles per square centimeter per hour at current densities ranging from 100 to 1000 μA cm⁻². For example, interpolating our data we find that with an ALA concentration of 2% in the donor chamber, a current density of 0.255 mA cm⁻² transports 0.065 μmol cm⁻² ALA across the SC in 10 min (conditions of Rhodes et al. (1997), J. Invest. Dermatol. 108, 87–91). For passive diffusion we find that a 5 h topical application of 20% ALA results in the transport of 0.05 μmol cm⁻². Thus, the amount of ALA that passively diffuses through the SC in several hours, leading to therapeutic levels of protoporphyrin IX (PpIX) in the epidermis, can be delivered by iontophoresis in 10 min or less. However, because the formation of sufficient PpIX also requires several hours and also because the SC overlying skin lesions such as basal cell carcinoma (BCC) is not intact, the clinical benefit of topical ALA delivery by iontophoresis for PDT of BCC is yet to be established.