diff --git a/aurweb/asgi.py b/aurweb/asgi.py
index ad0b7ca0..fa2526ed 100644
--- a/aurweb/asgi.py
+++ b/aurweb/asgi.py
@@ -74,6 +74,10 @@ async def app_startup():
     if not session_secret:
         raise Exception("[fastapi] session_secret must not be empty")
 
+    if not os.environ.get("PROMETHEUS_MULTIPROC_DIR", None):
+        logger.warning("$PROMETHEUS_MULTIPROC_DIR is not set, the /metrics "
+                       "endpoint is disabled.")
+
     app.mount("/static/css",
               StaticFiles(directory="web/html/css"),
               name="static_css")
diff --git a/aurweb/routers/html.py b/aurweb/routers/html.py
index b9d291d2..d31a32c7 100644
--- a/aurweb/routers/html.py
+++ b/aurweb/routers/html.py
@@ -13,7 +13,7 @@ from sqlalchemy import and_, case, or_
 import aurweb.config
 import aurweb.models.package_request
 
-from aurweb import cookies, db, models, time, util
+from aurweb import cookies, db, logging, models, time, util
 from aurweb.cache import db_count_cache
 from aurweb.exceptions import handle_form_exceptions
 from aurweb.models.account_type import TRUSTED_USER_AND_DEV_ID, TRUSTED_USER_ID
@@ -21,6 +21,7 @@ from aurweb.models.package_request import PENDING_ID
 from aurweb.packages.util import query_notified, query_voted, updated_packages
 from aurweb.templates import make_context, render_template
 
+logger = logging.get_logger(__name__)
 router = APIRouter()
 
 
@@ -230,9 +231,12 @@ async def archive_sha256(request: Request, archive: str):
 
 @router.get("/metrics")
 async def metrics(request: Request):
+    if not os.environ.get("PROMETHEUS_MULTIPROC_DIR", None):
+        return Response("Prometheus metrics are not enabled.",
+                        status_code=HTTPStatus.SERVICE_UNAVAILABLE)
+
     registry = CollectorRegistry()
-    if os.environ.get("PROMETHEUS_MULTIPROC_DIR", None):  # pragma: no cover
-        multiprocess.MultiProcessCollector(registry)
+    multiprocess.MultiProcessCollector(registry)
     data = generate_latest(registry)
     headers = {
         "Content-Type": CONTENT_TYPE_LATEST,
diff --git a/test/test_asgi.py b/test/test_asgi.py
index 667ae871..c693a3a9 100644
--- a/test/test_asgi.py
+++ b/test/test_asgi.py
@@ -104,6 +104,17 @@ async def test_asgi_app_unsupported_backends():
             await aurweb.asgi.app_startup()
 
 
+@pytest.mark.asyncio
+async def test_asgi_app_disabled_metrics(caplog: pytest.LogCaptureFixture):
+    env = {"PROMETHEUS_MULTIPROC_DIR": str()}
+    with mock.patch.dict(os.environ, env):
+        await aurweb.asgi.app_startup()
+
+    expected = ("$PROMETHEUS_MULTIPROC_DIR is not set, the /metrics "
+                "endpoint is disabled.")
+    assert expected in caplog.text
+
+
 @pytest.fixture
 def use_traceback():
     config_getboolean = aurweb.config.getboolean
diff --git a/test/test_html.py b/test/test_html.py
index b97d3571..ffe2a9f2 100644
--- a/test/test_html.py
+++ b/test/test_html.py
@@ -160,12 +160,23 @@ def test_archive_sig_404(client: TestClient):
 
 
 def test_metrics(client: TestClient):
-    with client as request:
-        resp = request.get("/metrics")
+    with tempfile.TemporaryDirectory() as tmpdir:
+        env = {"PROMETHEUS_MULTIPROC_DIR": tmpdir}
+        with mock.patch.dict(os.environ, env):
+            with client as request:
+                resp = request.get("/metrics")
     assert resp.status_code == int(HTTPStatus.OK)
     assert resp.headers.get("Content-Type").startswith("text/plain")
 
 
+def test_disabled_metrics(client: TestClient):
+    env = {"PROMETHEUS_MULTIPROC_DIR": str()}
+    with mock.patch.dict(os.environ, env):
+        with client as request:
+            resp = request.get("/metrics")
+    assert resp.status_code == int(HTTPStatus.SERVICE_UNAVAILABLE)
+
+
 def test_rtl(client: TestClient):
     responses = {}
     expected = [