BACKGROUND: Migraine headache affects approximately 9 million Americans. It is a chronic disease and is often associated with a high rate of disability. OBJECTIVE: The purpose of this research was to develop a model for predicting the incident number of migraine cases in the Idaho Medicaid population over a 5-year period. METHODS: Incident migraine cases between January 1994 and December 1998 were identified from the Idaho Medicaid claims database using migraine-specific ICD-9-CM codes or migraine-specific pharmacy claims. Data were collected on the total number of new migraine cases according to age, race, gender, and year. A negative binomial model was developed to predict incident migraine cases from the other variables. All model independents were treated as categorical variables. Two hundred sixty-six Medicaid recipients between the ages of 5 and 104 were included in the analysis. RESULTS: A test for overdispersion revealed that use of a negative binomial model was appropriate. Age, race, gender, and year were all determined to be significant effects by likelihood-ratio test. The greatest incidence of migraine was observed among persons 30-34 years of age. Controlling for other effects, the incidence of migraine fell with increasing age >34 or decreasing age <30. The estimated mean number of new cases was greater among Caucasians than non-Caucasians (46.2 vs. 3.7) and was greater among females than males (41.9 vs. 8.0). The estimated incidence of migraine was lowest in 1994 and generally rose each year from 1996 onward. Using a pseudo-R2 measure developed by Nagelkerke, the model explained 87.2% of the variance in incident migraine cases. CONCLUSIONS: The findings of this investigation with regard to age, race, and gender are consistent with those of previous studies. Our estimates for the impact of time on the incidence of migraine suggest that costs associated with migraine treatment will continue to rise for the Idaho Medicaid program.