Repetitive behaviours of five autistic and five intellectually handicapped children were observed under both fluorescent and incandescent lighting conditions. The findings of this study support the hypothesis that autistic children engage in a significantly greater frequency of stereotypies under fluorescent as compared to incandescent lighting, and that there is no significant difference in the frequency of stereotypic behaviours among intellectually handicapped children when exposed to the different lighting conditions. Implications of the findings are discussed in terms of the lighting requirements of therapeutic settings and the differential diagnosis of childhood autism.