diff --git a/src/async_impl/client.rs b/src/async_impl/client.rs
index 383b3b3..092d30c 100644
--- a/src/async_impl/client.rs
+++ b/src/async_impl/client.rs
@@ -78,6 +78,8 @@ struct Config {
     tls: TlsBackend,
     http2_only: bool,
     http1_title_case_headers: bool,
+    http2_initial_stream_window_size: Option<u32>,
+    http2_initial_connection_window_size: Option<u32>,
     local_address: Option<IpAddr>,
     nodelay: bool,
     #[cfg(feature = "cookies")]
@@ -116,6 +118,8 @@ impl ClientBuilder {
                 tls: TlsBackend::default(),
                 http2_only: false,
                 http1_title_case_headers: false,
+                http2_initial_stream_window_size: None,
+                http2_initial_connection_window_size: None,
                 local_address: None,
                 nodelay: false,
                 #[cfg(feature = "cookies")]
@@ -215,6 +219,13 @@ impl ClientBuilder {
             builder.http2_only(true);
         }
 
+        if let Some(http2_initial_stream_window_size) = config.http2_initial_stream_window_size {
+            builder.http2_initial_stream_window_size(http2_initial_stream_window_size);
+        }
+        if let Some(http2_initial_connection_window_size) = config.http2_initial_connection_window_size {
+            builder.http2_initial_connection_window_size(http2_initial_connection_window_size);
+        }
+
         builder.max_idle_per_host(config.max_idle_per_host);
 
         if config.http1_title_case_headers {
@@ -441,6 +452,22 @@ impl ClientBuilder {
         self
     }
 
+    /// Sets the `SETTINGS_INITIAL_WINDOW_SIZE` option for HTTP2 stream-level flow control.
+    ///
+    /// Default is currently 65,535 but may change internally to optimize for common uses.
+    pub fn http2_initial_stream_window_size(mut self, sz: impl Into<Option<u32>>) -> ClientBuilder {
+        self.config.http2_initial_stream_window_size = sz.into();
+        self
+    }
+
+    /// Sets the max connection-level flow control for HTTP2
+    ///
+    /// Default is currently 65,535 but may change internally to optimize for common uses.
+    pub fn http2_initial_connection_window_size(mut self, sz: impl Into<Option<u32>>) -> ClientBuilder {
+        self.config.http2_initial_connection_window_size = sz.into();
+        self
+    }
+
     // TCP options
 
     /// Set that all sockets have `SO_NODELAY` set to `true`.
diff --git a/src/blocking/client.rs b/src/blocking/client.rs
index 7d1b14d..9fedf53 100644
--- a/src/blocking/client.rs
+++ b/src/blocking/client.rs
@@ -261,6 +261,20 @@ impl ClientBuilder {
         self.with_inner(|inner| inner.http2_prior_knowledge())
     }
 
+    /// Sets the `SETTINGS_INITIAL_WINDOW_SIZE` option for HTTP2 stream-level flow control.
+    ///
+    /// Default is currently 65,535 but may change internally to optimize for common uses.
+    pub fn http2_initial_stream_window_size(self, sz: impl Into<Option<u32>>) -> ClientBuilder {
+        self.with_inner(|inner| inner.http2_initial_stream_window_size(sz))
+    }
+
+    /// Sets the max connection-level flow control for HTTP2
+    ///
+    /// Default is currently 65,535 but may change internally to optimize for common uses.
+    pub fn http2_initial_connection_window_size(self, sz: impl Into<Option<u32>>) -> ClientBuilder {
+        self.with_inner(|inner| inner.http2_initial_connection_window_size(sz))
+    }
+
     // TCP options
 
     /// Set that all sockets have `SO_NODELAY` set to `true`.