Background.: Organ transplant recipients on long-term immunosuppressive therapy are at increased risk of non-melanoma skin lesions. Repeated field photodynamic therapy using topical methyl aminolevulinate (MAL) may have potential as a preventive treatment. Methods.: This open randomized, intrapatient, comparative, multicenter study included 81 transplant recipients with 889 lesions (90% actinic keratoses (AK)]. In each patient, the study treatment was initially administered to one 50 cm area on the face, scalp, neck, trunk, or extremities (n=476 lesions) twice (1 week apart), with additional single treatments at 3, 9, and 15 months. On each occasion, the area was debrided gently and MAL cream (160 mg/g) applied for 3 hr, before illumination with noncoherent red light (630 nm, 37 J/cm). The control, 50 cm area (n=413 lesions) received lesion-specific treatment (83% cryotherapy) at baseline and 3, 9, and 15 months. Additionally, all visible lesions were given lesion-specific treatment 21 and 27 months in both treatment and control areas. Results.: At 3 months, MAL photodynamic therapy significantly reduced the occurrence of new lesions (65 vs. 103 lesions in the control area; P=0.01), mainly AK (46% reduction; 43 vs. 80; P=0.006). This effect was not significant at 27 months (253 vs. 312; P=0.06). Hypopigmentation, as assessed by the investigator, was less evident in the treatment than control areas (16% vs. 51% of patients; P<0.001) at 27 months. Conclusion.: Our results suggest that repeated field photodynamic therapy using topical MAL may prevent new AK in transplant recipients although further studies are needed.