diff --git a/.ci/build_cygwin.sh b/.ci/build_cygwin.sh
new file mode 100644
index 000000000..d621a5336
--- /dev/null
+++ b/.ci/build_cygwin.sh
@@ -0,0 +1,8 @@
+#!/bin/bash
+
+set -e
+
+python3 -m coverage erase
+make clean
+CFLAGS="-coverage -Werror=implicit-function-declaration" python3 -m pip install -v --global-option="build_ext" .
+python3 selftest.py
diff --git a/.ci/install.sh b/.ci/install.sh
index efc57a641..ec427ff27 100755
--- a/.ci/install.sh
+++ b/.ci/install.sh
@@ -13,17 +13,20 @@ aptget_update()
         return 1
     fi
 }
-aptget_update || aptget_update retry || aptget_update retry
+if [[ $(uname) != CYGWIN* ]]; then
+    aptget_update || aptget_update retry || aptget_update retry
+fi
 
 set -e
 
-sudo apt-get -qq install libfreetype6-dev liblcms2-dev python3-tk\
-                         ghostscript libffi-dev libjpeg-turbo-progs libopenjp2-7-dev\
-                         cmake meson imagemagick libharfbuzz-dev libfribidi-dev
+if [[ $(uname) != CYGWIN* ]]; then
+    sudo apt-get -qq install libfreetype6-dev liblcms2-dev python3-tk\
+                             ghostscript libffi-dev libjpeg-turbo-progs libopenjp2-7-dev\
+                             cmake meson imagemagick libharfbuzz-dev libfribidi-dev
+    python3 -m pip install --upgrade pip
+fi
 
-python3 -m pip install --upgrade pip
 python3 -m pip install --upgrade wheel
-PYTHONOPTIMIZE=0 python3 -m pip install cffi
 python3 -m pip install coverage
 python3 -m pip install defusedxml
 python3 -m pip install olefile
@@ -32,24 +35,30 @@ python3 -m pip install -U pytest-cov
 python3 -m pip install -U pytest-timeout
 python3 -m pip install pyroma
 python3 -m pip install test-image-results
-python3 -m pip install numpy
 
-# PyQt5 doesn't support PyPy3
-if [[ $GHA_PYTHON_VERSION == 3.* ]]; then
-  # arm64, ppc64le, s390x CPUs:
-  # "ERROR: Could not find a version that satisfies the requirement pyqt5"
-    sudo apt-get -qq install libxcb-xinerama0 pyqt5-dev-tools
-    python3 -m pip install pyqt5
+if [[ $(uname) != CYGWIN* ]]; then
+    PYTHONOPTIMIZE=0 python3 -m pip install cffi
+    python3 -m pip install numpy
+
+    # PyQt5 doesn't support PyPy3
+    if [[ $GHA_PYTHON_VERSION == 3.* ]]; then
+      # arm64, ppc64le, s390x CPUs:
+      # "ERROR: Could not find a version that satisfies the requirement pyqt5"
+        sudo apt-get -qq install libxcb-xinerama0 pyqt5-dev-tools
+        python3 -m pip install pyqt5
+    fi
+
+    # webp
+    pushd depends && ./install_webp.sh && popd
+
+    # libimagequant
+    pushd depends && ./install_imagequant.sh && popd
+
+    # raqm
+    pushd depends && ./install_raqm.sh && popd
+
+    # extra test images
+    pushd depends && ./install_extra_test_images.sh && popd
+else
+    cd depends && ./install_extra_test_images.sh && cd ..
 fi
-
-# webp
-pushd depends && ./install_webp.sh && popd
-
-# libimagequant
-pushd depends && ./install_imagequant.sh && popd
-
-# raqm
-pushd depends && ./install_raqm.sh && popd
-
-# extra test images
-pushd depends && ./install_extra_test_images.sh && popd
diff --git a/.github/workflows/test-cygwin.yml b/.github/workflows/test-cygwin.yml
index f4fbfd1f8..6e2dc5b59 100644
--- a/.github/workflows/test-cygwin.yml
+++ b/.github/workflows/test-cygwin.yml
@@ -41,24 +41,15 @@ jobs:
         run: |
           bash.exe -c "python3 .github/workflows/system-info.py"
 
-      - name: Ensure Python dependencies are installed
+      - name: Install dependencies
         run: |
-          bash.exe -c "python3 -m pip install pip wheel setuptools"
-          bash.exe -c "python3 -m pip install -r requirements.txt"
+          bash.exe .ci/install.sh
 
-      - name: Download extra test images
+      - name: Build
         run: |
-          bash.exe -c "cd depends; dash install_extra_test_images.sh"
+          bash.exe .ci/build_cygwin.sh
 
-      - name: Build Pillow
-        run: |
-          bash.exe -c "python3 setup.py bdist_wheel"
-
-      - name: Install Pillow
-        run: |
-          bash.exe -c "python3 -m pip install dist/*.whl"
-
-      - name: Test Pillow
+      - name: Test
         run: |
           bash.exe xvfb-run -s '-screen 0 1024x768x24' .ci/test.sh