diff --git a/src/HttpServer.cc b/src/HttpServer.cc
index 10582c62..df903e3a 100644
--- a/src/HttpServer.cc
+++ b/src/HttpServer.cc
@@ -109,6 +109,8 @@ Config::Config()
       num_worker(1),
       max_concurrent_streams(100),
       header_table_size(-1),
+      window_bits(-1),
+      connection_window_bits(-1),
       port(0),
       verbose(false),
       daemon(false),
@@ -845,11 +847,28 @@ int Http2Handler::connection_made() {
     entry[niv].value = config->header_table_size;
     ++niv;
   }
+
+  if (config->window_bits != -1) {
+    entry[niv].settings_id = NGHTTP2_SETTINGS_INITIAL_WINDOW_SIZE;
+    entry[niv].value = (1 << config->window_bits) - 1;
+    ++niv;
+  }
+
   r = nghttp2_submit_settings(session_, NGHTTP2_FLAG_NONE, entry.data(), niv);
   if (r != 0) {
     return r;
   }
 
+  if (config->connection_window_bits != -1) {
+    r = nghttp2_submit_window_update(
+        session_, NGHTTP2_FLAG_NONE, 0,
+        (1 << config->connection_window_bits) - 1 -
+            NGHTTP2_INITIAL_CONNECTION_WINDOW_SIZE);
+    if (r != 0) {
+      return r;
+    }
+  }
+
   ev_timer_start(sessions_->get_loop(), &settings_timerev_);
 
   if (ssl_ && !nghttp2::ssl::check_http2_requirement(ssl_)) {
diff --git a/src/HttpServer.h b/src/HttpServer.h
index ff2b844f..151bae71 100644
--- a/src/HttpServer.h
+++ b/src/HttpServer.h
@@ -67,6 +67,8 @@ struct Config {
   size_t num_worker;
   size_t max_concurrent_streams;
   ssize_t header_table_size;
+  int window_bits;
+  int connection_window_bits;
   uint16_t port;
   bool verbose;
   bool daemon;
diff --git a/src/nghttpd.cc b/src/nghttpd.cc
index 8add1951..a2921e88 100644
--- a/src/nghttpd.cc
+++ b/src/nghttpd.cc
@@ -143,6 +143,11 @@ Options:
               Default: 1
   -e, --error-gzip
               Make error response gzipped.
+  -w, --window-bits=<N>
+              Sets the stream level initial window size to 2**<N>-1.
+  -W, --connection-window-bits=<N>
+              Sets  the  connection  level   initial  window  size  to
+              2**<N>-1.
   --dh-param-file=<PATH>
               Path to file that contains  DH parameters in PEM format.
               Without  this   option,  DHE   cipher  suites   are  not
@@ -202,6 +207,8 @@ int main(int argc, char **argv) {
         {"max-concurrent-streams", required_argument, nullptr, 'm'},
         {"workers", required_argument, nullptr, 'n'},
         {"error-gzip", no_argument, nullptr, 'e'},
+        {"window-bits", required_argument, nullptr, 'w'},
+        {"connection-window-bits", required_argument, nullptr, 'W'},
         {"no-tls", no_argument, &flag, 1},
         {"color", no_argument, &flag, 2},
         {"version", no_argument, &flag, 3},
@@ -214,7 +221,7 @@ int main(int argc, char **argv) {
         {"no-content-length", no_argument, &flag, 10},
         {nullptr, 0, nullptr, 0}};
     int option_index = 0;
-    int c = getopt_long(argc, argv, "DVb:c:d:ehm:n:p:va:", long_options,
+    int c = getopt_long(argc, argv, "DVb:c:d:ehm:n:p:va:w:W:", long_options,
                         &option_index);
     char *end;
     if (c == -1) {
@@ -281,6 +288,26 @@ int main(int argc, char **argv) {
         std::cerr << "-p: Bad option value: " << optarg << std::endl;
       }
       break;
+    case 'w':
+    case 'W': {
+      char *endptr;
+      errno = 0;
+      auto n = strtoul(optarg, &endptr, 10);
+      if (errno != 0 || *endptr != '\0' || n >= 31) {
+        std::cerr << "-" << static_cast<char>(c)
+                  << ": specify the integer in the range [0, 30], inclusive"
+                  << std::endl;
+        exit(EXIT_FAILURE);
+      }
+
+      if (c == 'w') {
+        config.window_bits = n;
+      } else {
+        config.connection_window_bits = n;
+      }
+
+      break;
+    }
     case '?':
       util::show_candidates(argv[optind - 1], long_options);
       exit(EXIT_FAILURE);