Medicaid Eligibility and Benefits Expansion Act of 1991 - Amends title XIX (Medicaid) of the Social Security Act to require States to provide Medicaid coverage to pregnant women and infants whose family income does not exceed 150 percent of the Federal poverty level.

Requires States to cover all individuals with family incomes up to 120 percent of the poverty level.

Considers certain pensions, annuities, and disability benefits as earnings in determining the continued Medicaid eligibility of disabled individuals whose earnings disqualify them for benefits under the Supplemental Security Income (SSI) program (title XVI of the Social Security Act).

Expands Medicaid coverage of children to include children who would be SSI-eligible if they were not in an institution.

Provides temporary Medicaid coverage for dependents of unemployed individuals.

Allows States to provide Medicaid coverage for routine annual screening for breast and colon cancer and instruction in healthy living.

Permits States to require individuals whose family income equals or exceeds 100 percent of the Federal poverty level to make coinsurance payments with respect to services provided under State Medicaid plans.

Prohibits coinsurance payments from exceeding two percent of the reasonable cost of providing the service.

Directs the Secretary of Health and Human Services to: (1) evaluate on a regular basis the cost and quality of services provided under Medicaid; and (2) report to specified congressional committees on overpriced or poor quality services.