Controversy continues as to whether sluggish cognitive tempo (SCT) is a subtype of attention-deficit/hyperactivity disorder (ADHD) or a distinct disorder. This study examined differences between these disorders in demographics, executive functioning (EF), impairment, and prior professional diagnoses to address the issue. There were 1,800 children 6 to 17 years of age of both sexes from various U.S. ethnic backgrounds who were divided into four groups: (a) high SCT but not ADHD ("N" = 41), (b) high ADHD but not SCT ("N" = 95), (c) high in both SCT and ADHD ("N" = 61), and (d) the control group ("N" = 1,603). Besides providing demographics, parents completed scales assessing ADHD and SCT symptoms, EF deficits, and psychosocial impairment and reported their child's history of professional diagnoses. SCT symptoms formed two distinct but interrelated factors separate from those for ADHD. SCT differed from ADHD in demographics (age, sex ratio, parental education, income). ADHD was associated with more severe and pervasive EF deficits than SCT, whereas SCT was chiefly associated with mild deficits in Self-Organization. ADHD contributed far more variance to EF deficits than did SCT. Both disorders were impairing, but ADHD was more severely and pervasively so than SCT, especially in Home-School domains; SCT was most impairing in Community-Leisure domains. Different patterns of comorbidity were evident between SCT and ADHD. SCT showed less comorbidity and was particularly associated with depression. SCT may comprise a distinct disorder from ADHD but both may coexist in 39% to 59% of cases each. (Contains 4 tables.)