A method of underwriting group disability insurance for a policy period includes collecting medical claims data for the group to be underwritten, where each medical claim being related to a particular employee of the group. Morbidity categories are provided that categorize the medical claims in the medical claims data. A conditional probability model is developed and applied to the morbidity categories for each employee in the group using his medical claims, thereby calculating the expected conditional probability for each employee of incurring a disability during the policy period. A further statistical model of the estimated cost of the disability is developed and applied based on the employees' morbidity categories from the medical claims data. For each employee, an estimate of the expected cost of incurring a disability given their morbidity categories is derived from his medical claims data. Combining the expected conditional probability for each employee incurring a disability during the policy period with the estimate of the expected cost of that disability gives an estimate of the group's total disability exposure. Thereby, the expected disability exposure is used to determine a premium amount for disability insurance coverage during the policy period for the group.