||This study compared females and males with an autism spectrum disorder (ASD) and normal intelligence to explore sex differences in onset and trajectory of ASD symptoms as well as developmental tasks and social functioning. Personality traits were also studied, as was the presence of psychiatric problems. Twenty-one females and 21 males with ASD between the ages of 13 and 35 years of age participated in the study. Although these 42 individuals are the "participants," their parents provided important information about development of symptoms and functioning that the individuals with ASD may not have recalled. Both participants and parents of participants completed comprehensive structured interview and rating scales that evaluated ASD symptoms and social-behavioral functioning. Parents also completed a symptom report form, and participants with ASD completed an IQ test to determine eligibility for the study and a personality test. Results show that parents of female participants noticed ASD symptoms at a later age than parents of males (e.g., sensory symptoms and abnormal social interactions) and reported these to professionals at a later age (i.e., around 7 years for females compared to 3 years, 9 months for males). Professionals, often the child's pediatricians, also showed a tendency to normalize the behaviors of females with ASD more often than males, and did not refer females as often for follow-up evaluations. Despite all of this, no differences were found in the ages at which an ASD diagnosis was made for both sexes, which was on average about 10 years, 9 months. Parents of females reported a more fluctuating symptom presentation than did parents of males, and also reported greater improvement over time. Despite this, female participants reported more distress over social difficulties than males, and this seemed to be consistent over time. Parents of females tended to notice their daughters having more frustration and anger than parents of males, and also observed them to engage more in self-injurious behaviors. Personality testing further showed that the older female participants endorsed more psychological problems than the younger females or the males, including problems with anxiety and depression.