PURPOSE: To describe the multimodal imaging characteristics of flat irregular pigment epithelial detachment (FIPED) in patients with chronic central serous chorioretinopathy and determine the risk factors for vascularized FIPED and to explore the activity of vascularized FIPED before and after half-dose photodynamic therapy. METHODS: Multimodal imaging data of 185 eyes of 155 consecutive patients with chronic central serous chorioretinopathy included spectral domain optical coherence tomography, fluorescein angiography, and indocyanine green angiography. Optical coherence tomography angiography was available for 56 eyes. Flat irregular PED was classified into two types based on indocyanine green angiography or optical coherence tomography angiography findings: avascular FIPED and vascularized FIPED. RESULTS: The avascular FIPED and vascularized FIPED were detected in 127 (68.6%) and 42 (22.7%) eyes, respectively. Age (P = 0.001), visual acuity (P = 0.048), subfoveal choroidal thickness (P = 0.032), height (P < 0.001) and width (P < 0.001) of FIPED, choriocapillary thickness (P = 0.015), and maximum vessel diameter (P = 0.009) beneath the FIPED were significantly different between avascular and vascularized FIPEDs. Old age was an independent risk factor for vascularized FIPED. On optical coherence tomography angiography, all vascularized FIPEDs manifested the pattern of mature choroidal neovascularization (CNV). After half-dose photodynamic therapy, vascularized FIPED remained stable without the reaccumulation of subretinal fluid at the last follow-up. CONCLUSION: In chronic central serous chorioretinopathy, vascularized FIPED was closely associated with Type I CNV. Old age was an independent risk factor for vascularized FIPED. Vascularized FIPED is suggested as "quiescent" CNV, and half-dose photodynamic therapy may be recommended as the first-line therapy in chronic central serous chorioretinopathy complicated with quiescent CNV, except when the activity of CNV becomes evident.