Depression is highly prevalent in children who have an Autism Spectrum Disorder (ASD), potentially confounding accurate diagnosis and treatment planning. Although information about the depressive status of a child is often collected from parents, there is evidence of distortion in parental assessments of their offspring's depression. This distortion was investigated in a sample of 132 mothers of boys with an ASD. Results indicated that, as predicted from previous studies, there was a significant correlation between the depressive state of the mothers and the ratings they gave for their sons' depression. However, in contradiction to the expected influence direction, mothers who were minimally depressed "under-estimated" their sons' depression when cross-validated by structured clinical interviews for depression conducted by a third party, rather than mothers who were more severely depressed "exaggerating" their sons' depression. Implications for clinical assessment of the presence of depression in boys with an ASD are discussed.