Study objectives: Prior research investigating co-occurring insomnia/obstructive sleep apnea (CIO) has mainly focused on comparing comorbid patients, obstructive sleep apnea (OSA), and insomnia (INS) to those with OSA alone. This approach is informative but omits the potentially interesting comparison of comorbid patients to those with INS alone. Our study used an incomplete factorial design, crossing OSA (present or absent) with INS (present or absent) to more clearly focus on the question, is comorbid INS an epiphenomenon of OSA or an independent disorder? Methods: Our study was an archival analysis from the database of a sleep center comparing consecutively diagnosed patients characterized as OSA or INS. A third group, CIO, was derived from the OSA group. Our study was conducted at an American Academy of Sleep Medicine-accredited sleep disorders center. We studied 299 patients, including 94 OSA, 97 INS, and 108 CIO. Patients ranged from ages 15 to 86. years. Results: Groups were compared on polysomnography (PSG), sleep pattern, sleep stages, sleep pathology, self-reported sleep concerns, and self-reported daytime functioning. From a consecutive group of OSA patients, we estimate the prevalence of CIO at 67.4%. Based mainly on multivariate analysis of covariance (MANCOVA) controlling for demographic differences between groups, we found few if any significant differences between CIO and INS alone or between CIO and OSA alone. Conclusions: The clinical presentation of CIO is indistinguishable from INS alone, both with respect to PSG findings and to self-reported sleep onset and sleep maintenance disturbance. We observed a weak relation between OSA severity and co-occurring INS. These data are consistent with the view that INS with co-occurring OSA is an independent, self-sustaining disorder. We hypothesized that in some unknown proportion of cases, OSA initially instigated the INS, but the INS was then perpetuated and reshaped by sleep concerns and self-defeating compensatory behaviors. © 2013 Elsevier B.V.