Background -Pythiosis is a life-threatening disease caused by Pythium insidiosum. Photodynamic therapy (PDT) is an alternative treatment to surgery that uses the interaction of a photosensitizer, light and molecular oxygen to cause cell death.Objectives -To evaluate the effect of PDT on the in vitro growth of P. insidiosum and in an in vivo model of pythiosis.Methods -For in vitro studies, two photosensitizers were evaluated: a haematoporphyrin derivative (Photogem â ) and a chlorine (Photodithazine â ). Amphotericin B was also evaluated, and the control group was treated with sterile saline solution. All experiments (PDT, porphyrin, chlorine and light alone, amphotericin B and saline solution) were performed as five replicates. For in vivo studies, six rabbits were inoculated with 20,000 zoospores of P. insidiosum, and an area of 1 cm 3 was treated using the same sensitizers. The PDT irradiation was performed using a laser emitting at 660 nm and a fluence of 200 J ⁄ cm 2 . Rabbits were clinically evaluated daily and histopathological analysis was performed 72 h after PDT.Results -For in vitro assays, inhibition rates for PDT ranged from 60 to 100% and showed better results in comparison to amphotericin B. For the in vivo assays, after PDT, histological analysis of lesions showed a lack of infection up to 1 cm in depth.Conclusions and clinical importance -In vitro and in vivo studies showed that PDT was effective in the inactivation of P. insidiosum and may represent a new approach to treating pythiosis. Accepted 13 July 2012 Sources of Funding: Fundaçã o de Amparo à Pesquisa do Estado de Sã o Paulo -FAPESP (grants 2008 ⁄ 03511-8 and 2010 ⁄ 02890-5) and FUNDUNESP (grant 42 ⁄ 11).

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