Primary insurance plan of a Medicare beneficiary that must pay for any medical care or services first before Medicare is sent a claim. MSP may involve aged or disabled patients who are under group health plans and cases related to end-stage renal disease, black lung disease, workers’ compensation, automobile accidents (e.g., medical no-fault and liability insurance), and individuals receiving benefits under the Department of Veterans Affairs and Medicare. For a Medicare patient suffering from end-stage renal disease (ESRD), MSP is the payer for the first 30 months that the beneficiary is entitled to benefits.