Objectives: To determine the effect on the visual outcome after macular hole s urgery when staining the internal limiting membrane (ILM) with indocyanine green (ICG) dye and to study the mechanism of the adverse effects. Patients and Metho ds: We studied 40 eyes of 38 patients with an idiopathic macular hole (size,< 0. 5 disc diameter; duration,< 12 months). The concentration, exposure time, and am ount of the ICG solution that was minimally required to make the ILM visible wer e determined. The patients were randomly divided into group 1 (20 eyes of 19 pat ients) who underwent ILM peeling without ICG staining, and group 2 (20 eyes of 19 patients) who underwent ILM peeling with ICG staining. Routine examinations were conducted during the 12-month follow-u p period. Multifocal electroretinogram, optical coherence tomography, and fluore scein angiography were performed on 31 eyes of 30 patients. Results: The macular hole was closed in all patients. Visual acuity was improved in both groups, but it was significantly better in group 1 (median, 0.85) than in group 2 (median, 0.60; P=0.02) after 12 months. The improvement of visual acuity in group 1 (loga rithm of the minimum angle of resolution log-MAR units SD, 0.82 0.19)was significantly better than that in group 2 (logMAR units, 0.67 0.21; P=0.30). T he multifocal electroretinogram and optical coherence tomographic findings were not significantly different in the 2 groups. Fluorescein angiogram showed only w eak hyperfluorescence at the macula in some patients of both groups. Conclusions : The results suggest that ICG staining should not be used as long as the visibi lity of the retinal surface is good. However, ICG staining may be acceptable at a low concentration when a clear view of the retinal surface is unattainable. Th e results of the multifocal electroretinogram, optical coherence tomography, and fluorescein angiography suggest that the differences in visual recovery were ca used not only by pigment epithelial cell damage or retinal toxic effect but also probably by the effect of ICG staining on ganglion cells and their axons. Objectives: To determine the effect on the visual outcome after macular hole s urgery when staining the internal limiting membrane (ILM) with indocyanine green (ICG) dye and to study the mechanism of the adverse effects. Patients and Metho ds: We studied 40 eyes of 38 patients with an idiopathic macular hole (size, <0.5 disc diameter; duration, <12 months). The concentration, exposure time, and am count of the ICG solution that was minimally required to make the ILM visible . The patients were randomly divided into group 1 (20 eyes of 19 patients) who underwent ILM peeling without ICG staining, and group 2 (20 eyes of 19 patients) who underwent ILM peeling with ICG staining. Routine examinations were conducted during the 12 -month follow-u p period. Multifocal electroretinogram, optical coherence tomography, and fluore scein angiography were performed on 31 eyes of 30 patients. Results: The macular hole was closed in all patients. ps, but it was significantly better in group 1 (median, 0.85) than in group 2 (median, 0.60; P = 0.02) after 12 months. The improvement of visual acuity in group 1 (log rithm of the minimum angle of resolution log -MAR units SD, 0.82 0.19) was significantly better than that in group 2 (logMAR units, 0.67 0.21; P = 0.30). T he multifocal electroretinogram and optical coherence tomographic findings were not significantly different in the 2 groups. Fluorescein angiogram showed only However, ICG staining may be acceptable at a low concentration when a clear view of the retinal surface is unattainable. Th e results of the multifocal electroretinogram, optical coherence tomography, and fluorescein angiography suggest that the differences in visual recovery were ca not only by pigment epithelialcell damage or retinal toxic effect but also probably by the effect of ICG staining on ganglion cells and their axons.