diff --git a/stdnum/no/orgnr.py b/stdnum/no/orgnr.py
index c3bc668b..2c1e13a4 100644
--- a/stdnum/no/orgnr.py
+++ b/stdnum/no/orgnr.py
@@ -21,12 +21,14 @@
 """Orgnr (Organisasjonsnummer, Norwegian organisation number).
 
 The Organisasjonsnummer is a 9-digit number with a straightforward check
-mechanism.
+mechanism. The first digit is 8 or 9 and the last digit is a check digit
+calculated using modulus 11.
 
 More information:
 
 * https://nn.wikipedia.org/wiki/Organisasjonsnummer
 * https://no.wikipedia.org/wiki/Organisasjonsnummer
+* https://www.brreg.no/om-oss/registrene-vare/om-enhetsregisteret/organisasjonsnummeret/
 
 >>> validate('988 077 917')
 '988077917'
@@ -34,6 +36,10 @@
 Traceback (most recent call last):
     ...
 InvalidChecksum: ...
+>>> validate('100 000 008')
+Traceback (most recent call last):
+    ...
+InvalidComponent: ...
 >>> format('988077917')
 '988 077 917'
 """
@@ -64,6 +70,8 @@ def validate(number: str) -> str:
         raise InvalidFormat()
     if len(number) != 9:
         raise InvalidLength()
+    if number[0] not in '89':
+        raise InvalidComponent('The number must start with 8 or 9.')
     if checksum(number) != 0:
         raise InvalidChecksum()
     return number
diff --git a/tests/test_no_orgnr.doctest b/tests/test_no_orgnr.doctest
new file mode 100644
index 00000000..d36ed00e
--- /dev/null
+++ b/tests/test_no_orgnr.doctest
@@ -0,0 +1,71 @@
+test_no_orgnr.doctest - more detailed doctests for stdnum.no.orgnr module
+
+Copyright (C) 2026 M J Anthony
+
+This library is free software; you can redistribute it and/or
+modify it under the terms of the GNU Lesser General Public
+License as published by the Free Software Foundation; either
+version 2.1 of the License, or (at your option) any later version.
+
+This library is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public
+License along with this library; if not, see <https://www.gnu.org/licenses/>.
+
+
+This file contains more detailed doctests for the stdnum.no.orgnr module. It
+tries to cover more corner cases and detailed functionality that is not
+really useful as module documentation.
+
+>>> from stdnum.no import orgnr
+
+
+These have been found online and should all be valid numbers.
+
+>>> numbers = '''
+...
+... 988 077 917
+... 965 920 358
+... 980 430 596
+... 998 772 680
+... 987 008 644
+... 917 313 008
+... 966 813 946
+... 982 930 057
+... 982 952 573
+... 987 989 297
+...
+... '''
+>>> [x for x in numbers.splitlines() if x and not orgnr.is_valid(x)]
+[]
+
+
+Norwegian organisation numbers always start with 8 or 9. Each of the
+following numbers has a valid modulus 11 check digit but an invalid first
+digit, so they must all be rejected.
+
+>>> numbers = '''
+...
+... 000 000 000
+... 100 000 008
+... 200 000 005
+... 300 000 002
+... 410 000 008
+... 500 000 007
+... 600 000 004
+... 700 000 001
+...
+... '''
+>>> [x for x in numbers.splitlines() if x and orgnr.is_valid(x)]
+[]
+
+
+Numbers that start with 8 or 9 with a correct check digit remain valid.
+
+>>> orgnr.validate('800 000 009')
+'800000009'
+>>> orgnr.validate('900 000 006')
+'900000006'