This study examined the effect on reading performance of a controversial treatment for a dysfunction of visual perception known as Scotopic sensitivity/lrlen Syndrome. The treatment, referred to as spectral modification, involved the use of colored transparent overlays for reading by four elementary school children in their actual school environments. The diagnostic, prescriptive and remediation procedures used in this study were originally developed by Helen Irlen, a California psychologist and researcher. Irlen conceptualized Scotopic Sensitivity Syndrome as a difficulty in visual processing of the printed page when perceived through full spectrum light. Spectral modification techniques were considered controversial in that they were employed prior to scientific validation of the syndrome. Previous research used pre-test/post-test experimental designs to test the effect of spectral modification on reading performance. The research problem addressed here was that the technique had not been systematically examined over time in the school setting, and from the practitioner's point of view. Sample selection involved pre-screening and screening phases. The pre-screening procedures of teacher recommendation and file review resulted in a pool of 26 children who were screened with the Irlen Differential Perceptual Interview Survey. Four subjects were selected who were in separate school settings, who demonstrated Irlen Syndrome to a significant extent, and whose profiles did not present variables which would compromise their participation. These four underwent comprehensive vision evaluations as a preliminary assessment procedure. A four-strand single-subject experimental design was used to generate data on subjects' rate and accuracy of oral reading, and comprehension of silent reading. Performance changes during experimental reading trials varied between the four, although three subjects exhibited positive performance change on one indicator. A supporting procedure was that subjects were pre and post-tested on the three performance indicators. Post-test results corroborated the changes demonstrated by three subjects Interview formats were employed to obtain qualitative data from teachers, parents, and the subjects themselves. Recommendations for practitioners intending to implement spectral modification techniques were generated from this data.