REPORT OF INDEPENDENT REGISTERED PUBLIC
ACCOUNTING FIRM
To the Board of Trustees of MFS Series Trust XVI and the
Shareholders of MFS Prudent Investor Fund:
In planning and performing our audit of the financial
statements of MFS Prudent Investor Fund (the Fund) (a
series of MFS Series Trust XVI) as of and for the period
ended June 30, 2019, in accordance with the standards of
the Public Company Accounting Oversight Board (United
States), we considered the Fund?s internal control over
financial reporting, including controls over safeguarding
securities, as a basis for designing our auditing
procedures for the purpose of expressing our opinion on
the financial statements and to comply with the
requirements of Form N-CEN, but not for the purpose of
expressing an opinion on the effectiveness of the Fund?s
internal control over financial reporting.  Accordingly, we
express no such opinion.
The management of the Fund is responsible for
establishing and maintaining effective internal control
over financial reporting.  In fulfilling this responsibility,
estimates and judgments by management are required to
assess the expected benefits and related costs of
controls.  A fund?s internal control over financial reporting
is a process designed to provide reasonable assurance
regarding the reliability of financial reporting and the
preparation of financial statements for external purposes
in accordance with generally accepted accounting
principles.  A fund?s internal control over financial
reporting includes those policies and procedures that (1)
pertain to the maintenance of records that, in reasonable
detail, accurately and fairly reflect the transactions and
dispositions of the assets of the fund; (2) provide
reasonable assurance that transactions are recorded as
necessary to permit preparation of financial statements in
accordance with generally accepted accounting principles,
and that receipts and expenditures of the fund are being
made only in accordance with authorizations of
management of the fund and trustees of the trust; and
(3) provide reasonable assurance regarding prevention or
timely detection of unauthorized acquisition, use, or
disposition of a fund?s assets that could have a material
effect on the financial statements.
Because of its inherent limitations, internal control over
financial reporting may not prevent or detect
misstatements.  Also, projections of any evaluation of
effectiveness to future periods are subject to the risk that
controls may become inadequate because of changes in
conditions or that the degree of compliance with the
policies or procedures may deteriorate.
A deficiency in internal control over financial reporting
exists when the design or operation of a control does not
allow management or employees, in the normal course of
performing their assigned functions, to prevent or detect
misstatements on a timely basis.  A material weakness is
a deficiency, or a combination of deficiencies, in internal
control over financial reporting, such that there is a
reasonable possibility that a material misstatement of a
fund?s annual or interim financial statements will not be
prevented or detected on a timely basis.
Our consideration of the Fund?s internal control over
financial reporting was for the limited purpose described
in the first paragraph and would not necessarily disclose
all deficiencies in internal control that might be material
weaknesses under standards established by the Public
Company Accounting Oversight Board (United States).
However, we noted no deficiencies in the Fund?s internal
control over financial reporting and its operation,
including controls for safeguarding securities, that we
consider to be a material weakness, as defined above, as
of June 30, 2019.
This report is intended solely for the information and use
of management, the Board of Trustees of MFS Series
Trust XVI and the Securities and Exchange Commission
and is not intended to be and should not be used by
anyone other than these specified parties.

DELOITTE & TOUCHE LLP

Boston, Massachusetts
August 16, 2019