WASHINGTON, May 1 (Reuters) - UnitedHealth ( UNH ) CEO

Andrew Witty began his first of two scheduled testimonies in

front of Congressional panels on Wednesday, answering questions

from lawmakers about the recent cyberattack at the company's

technology unit that impacted almost all patients and providers.

The

hack

at Change Healthcare, a provider of healthcare billing and

data systems that processes about 50% of U.S. medical claims, on

Feb. 21 caused widespread disruption in payment to doctors and

health facilities.

Health and personal data of a "substantial proportion" of

Americans were stolen, UnitedHealth ( UNH ) said last week.

In letters to the congressional committees Witty will appear

before, the American Hospital Association said an internally

survey of its members found that 94% of hospitals reported

damage to cash flow and more than half reported "significant or

serious" financial damage due to Change's inability to process

claims.

Hackers breached UnitedHealth's ( UNH ) tech unit on Feb. 12 by

using stolen login credentials that gave them remote access to

its network, Witty will tell the Senate Finance Committee on

Wednesday morning and the House Energy Subcommittee on Oversight

and Investigations in the afternoon.

In a written testimony published earlier this week, Witty

said UnitedHealth ( UNH ), the largest U.S. health insurer, has been

working with the FBI and prominent cybersecurity firms to

investigate the hack.