diff --git a/CHANGES/386.feature b/CHANGES/386.feature
new file mode 100644
index 000000000..a44021b7c
--- /dev/null
+++ b/CHANGES/386.feature
@@ -0,0 +1 @@
+Convert host to lowercase on URL building.
diff --git a/yarl/_url.py b/yarl/_url.py
index 74d0dbd2a..563b53d2a 100644
--- a/yarl/_url.py
+++ b/yarl/_url.py
@@ -706,6 +706,7 @@ def _encode_host(cls, host):
                 ip, sep, zone = host.partition("%")
                 ip = ip_address(ip)
             except ValueError:
+                host = host.lower()
                 # IDNA encoding is slow,
                 # skip it for ASCII-only strings
                 # Don't move the check into _idna_encode() helper
@@ -729,6 +730,8 @@ def _encode_host(cls, host):
                 ip, sep, zone = host.partition("%")
                 ip = ip_address(ip)
             except ValueError:
+                host = host.lower()
+
                 for char in host:
                     if char > "\x7f":
                         break