Aims: To evaluate the impact of headache in adults with masticatory myofascial pain (MMP) on the outcome variables clinical pain (ie, self-reported pain intensity and pressure pain sensitivity), sleep quality, and pain catastrophizing. Methods: A total of 97 patients with MMP were diagnosed with co-existing headache (MMPH group, n = 50) or without headache (MMP group, n = 47) according to the Research Diagnostic Criteria for Temporomandibular Disorders (RDC/ TMD). The outcome parameters were the Pittsburgh Sleep Quality Index (PSQI); the Catastrophizing Thoughts subscale of the Pain-Related Self-Statement Scale (PRSS- C); pressure pain thresholds (PPTs) of the masseter and anterior temporalis muscles; and self-reported facial pain intensity measured on a 0- to 10-cm visual analog scale (VAS). Student t test for independent samples (α = 1.2%) and factorial analysis of variance (ANOVA) (α = 5%) were used to analyze the data. Results: The MMPH group showed significantly impaired sleep quality (mean ± standard deviation [SD] PSQI score 9.1 ± 3.5) compared with the MMP group (7.2 ± 3.4; P = .008). Subscale scores on the PRSS-C were significantly higher in the MMPH (2.1 ± 1.2) than in the MMP group (1.6 ± 1.4, uncorrected P = .048). Also, the PPTs (kgf/cm2) of the masseter and anterior temporalis muscles were significantly lower in the MMPH group (1.52 ± 0.53; 1.29 ± 0.43, respectively) than in the MMP group (2.09 ± 0.73; 1.70 ± 0.68, respectively; P .001), with no differences in self-reported facial pain intensity. Factorial analyses further indicated that chronic migraine was associated with poorer sleep quality (P = .003) and that tension-type headache patients had lower PPTs in the anterior temporalis muscle (P = .041) in comparison with nonheadache patients. Conclusion: Co-existence of headache further exacerbates clinical characteristics in patients with painful TMD, which implies involvement of common mechanisms and pathways of vulnerability in these patients. Keywords: catastrophization, comorbidity, headache, sleep, temporomandibular joint disorders