C172 pilot reported an near midair collision with another aircraft in the vicinity of pdx.

Attributes
ACN	1593590
Time
Date	201811
Local Time Of Day	1201-1800
Place
Locale Reference	P80.TRACON
State Reference	OR
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Skyhawk 172/Cutlass 172
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	Other Survey grid
Flight Plan	VFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Person 1
Function	Pilot Flying Single Pilot
Qualification	Flight Crew Commercial Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 100 Flight Crew Total 400 Flight Crew Type 320
Events
Anomaly	Conflict NMAC
Miss Distance	Horizontal 200 Vertical 100

Narrative:

Aircraft of unknown type overtook me. In cruise; I looked out my right window and saw the aircraft approaching from my 4 o'clock at a range of 200 feet; approximately 100 feet below me. Before I ever had the chance to take evasive action; the aircraft had already passed behind me; nearly striking my empennage with approximately 50 feet between the two aircraft.

Original NASA ASRS Text

Title: C172 pilot reported an NMAC with another aircraft in the vicinity of PDX.

Narrative: Aircraft of unknown type overtook me. In cruise; I looked out my right window and saw the aircraft approaching from my 4 o'clock at a range of 200 feet; approximately 100 feet below me. Before I ever had the chance to take evasive action; the aircraft had already passed behind me; nearly striking my empennage with approximately 50 feet between the two aircraft.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.