We examined factors related to subjective quality of life (QoL) of adults with autism spectrum disorder (ASD) aged 25–55 (n = 60), using the World Health Organization Quality of Life measure (WHOQOL-BREF). We used three different assessment methods: adult self-report, maternal proxy-report, and maternal report. Reliability analysis showed that adults with ASD rated their own QoL reliably. QoL scores derived from adult self-reports were more closely related to those from maternal proxy-report than from maternal report. Subjective factors such as perceived stress and having been bullied frequently were associated with QoL based on adult self-reports. In contrast, level of independence in daily activities and physical health were significant predictors of maternal reports of their son or daughter’s QoL.