47,XXY/Klinefelter syndrome is the most common sex chromosomal aneuploidy, yet 64% of males with this condition go undiagnosed. 48,XXYY is less common and there is less known about the diagnosis. The objective of this study is to describe the diagnosis experiences of parents of males with 47,XXY and 48,XXYY. Parents of 89 males with 47,XXY and 76 males with 48,XXYY completed a survey that gathered data about their experiences leading to a diagnosis, including the current age of the child, age at diagnosis, reasons for initial concern, and the specialists providing the diagnosis. In the 47,XXY cohort diagnosed postnatally, 59% presented with developmental delay, with a mean age at first parental concern of 5.2 years and mean age of diagnosis at 10.0 years. The remaining 41% presented with endocrinologic issues with a mean age at first concern of 19.1 years and mean age of diagnosis at 21.1 years. In the 48,XXYY group, 93% presented with developmental delay, with mean age at first parental concern of 2.4 years and mean age of diagnosis at 7.6 years. Hence, the average time from initial parental concern to diagnosis of 47,XXY or 48,XXYY ranges from 2 to 5 years, with those presenting with developmental issues having a longer lag to diagnosis compared to those presenting with endocrinologic issues. Increased awareness of the developmental, psychological, and medical features of 47,XXY and 48,XXYY is important to facilitate timely diagnosis and initiation of appropriate screenings and treatments that are important for optimal outcomes. © 2013 Wiley Periodicals, Inc.