OBJECTIVES: Evaluate the prevalence and costs of commonly reported insomnia comorbidities. METHODS: A database of US employees from 2001-2006 was used to identify subjects with insomnia (based on the International Classification of Diseases–9 [ICD-9] codes or prescription for a hypnotic agent) and control employees (3:1) (matched by demographics, job information, and geographic region). Direct medical costs (inflated to 2006 dollars) for each comorbid condition, based on ICD-9 codes from the Agency for Health Research and Quality 261 Specific Categories, were analyzed for the 12 months after insomnia diagnosis. Prevalence comparisons were calculated using z-scores of log odds ratios (Woolf method), and the average cost (for the entire cohort) comparisons were calculated using Sattherthwaite t-tests. RESULTS: A total of 12,308 employees with insomnia and 36,924 matched controls were analyzed. Results are presented for each comorbid condition as: (% insomnia prevalence: % control prevalence, insomnia costs: control costs). Mental Disorders: Dissociative/Personality Disorders (15.88%:3.32%, $34:$6); Affective Disorders (14.54%:2.92%, $125:$17); Substance-Related Mental Disorders (2.00%:0.80%, $7:$1); Alcohol-Related Mental Disorders (0.89%:0.22%, $18:$3); Other Mental Conditions (15.17%:4.27%, $44:$11); Other Psychoses (0.24%:0.05%, $2:$0). Respiratory System: Asthma (6.24%:3.04%, $20:$8); COPD and Bronchiectasis (4.14%:2.23%, $10:$5). Nervous System Sense Organs: Headache including Migraine (13.07%:5.16%, $54:$16); Dizziness/Vertigo (4.95%:2.35%, $16:$8); Hereditary/Degenerative Nervous System Condition (1.09%:0.22%, $5:$1); Other Nervous System Disorders (34.20%:5.01%, $168:$40). Injury/Poisoning: Sprains and Strains (13.40%:7.52%, $69:$31); Fracture-Lower Limb (1.58%:0.95%, $26:$8); Fracture-Upper Limb (1.26%:0.71%, $15:$7). Other Conditions: Rheumatoid Arthritis (1.31%:0.52%, $19:$7); Malaise and Fatigue (15.20%:5.81%, $14:$5); Syncope (1.59%:0.72%, $14:$5); Chronic Renal Failure (0.50%:0.13%, $74:$7); Breast Cancer (1.20%:0.67%, $112:$26). Overall, employees in the insomnia cohort had significantly higher prevalence (80.8%) and costs (47.5%) for the 261 conditions compared with the matched controls (p<0.05). CONCLUSION: Employees with insomnia have more prevalent comorbid conditions than subjects without insomnia. From an insurer's perspective, this increased burden for insomnia sufferers is also associated with higher costs.