We studied the nature and extent of comorbidity of chronic frequent headache (CFH) in the general population and the influence of CFH and comorbidity on quality of life. Subjects with CFH (headache on >14 days/month) were identified in a general health survey. We sent a second questionnaire including questions on comorbidity and quality of life to subjects with CFH and subjects with infrequent headache (IH) (1-4 days/month). We recoded comorbidity by using the Cumulative Illness Rating Scale (CIRS) and measured quality of life with the RAND-36, a Dutch version of Short Form-36. CFH subjects (n = 176) had higher comorbidity scores than the IH subjects (n = 141). Mean CIRS scores were 2.94 for CFH and 1.55 for IH [mean difference 1.40, 95% confidence interval (CI) 0.91, 1.89]. The mean number of categories selected was 1.92 in CFH and 1.10 in IH (mean difference 0.82, 95% CI 0.54, 1.11). Fifty percent of CFH subjects had a comorbidity severity level of at least 2, indicating disorders requiring daily medication, compared with 28% of IH subjects (mean difference 22%, 95% CI 12, 33). CFH subjects had more musculoskeletal, gastrointestinal, psychiatric and endocrine/breast pathology than IH subjects. Quality of life in CFH subjects was lower than that of IH subjects in all domains of the RAND-36. Both headache frequency and CIRS score had a negative influence on all domains. We conclude that patients with CFH have more comorbid disorders than patients with infrequent headaches. Many CFH patients have a comorbid chronic condition requiring daily medication. Both high headache frequency and comorbidity contribute to the low quality of life in these patients.