diff --git a/openssl-sys/src/lib.rs b/openssl-sys/src/lib.rs
index 27ef7e914c82f5de1d1927e68b80f464e1991828..d67d37409999778d587282e4448197d6a5953ac6 100644
--- a/openssl-sys/src/lib.rs
+++ b/openssl-sys/src/lib.rs
@@ -1655,6 +1655,7 @@ extern "C" {
 
     pub fn ASN1_INTEGER_get(dest: *const ASN1_INTEGER) -> c_long;
     pub fn ASN1_INTEGER_set(dest: *mut ASN1_INTEGER, value: c_long) -> c_int;
+    pub fn ASN1_INTEGER_to_BN(ai: *const ASN1_INTEGER, bn: *mut BIGNUM) -> *mut BIGNUM;
     pub fn ASN1_GENERALIZEDTIME_free(tm: *mut ASN1_GENERALIZEDTIME);
     pub fn ASN1_GENERALIZEDTIME_print(b: *mut BIO, tm: *const ASN1_GENERALIZEDTIME) -> c_int;
     pub fn ASN1_STRING_type_new(ty: c_int) -> *mut ASN1_STRING;
diff --git a/openssl/src/asn1.rs b/openssl/src/asn1.rs
index d129235ae633dad11fe990b6359de76ead0b6fc9..f6917aae2de647266fa9deeea4270b31a6747cfb 100644
--- a/openssl/src/asn1.rs
+++ b/openssl/src/asn1.rs
@@ -26,7 +26,7 @@
 //! ```
 use ffi;
 use foreign_types::{ForeignType, ForeignTypeRef};
-use libc::{c_long, c_char, c_int};
+use libc::{c_char, c_int, c_long};
 use std::fmt;
 use std::ptr;
 use std::slice;
@@ -34,6 +34,7 @@ use std::str;
 
 use {cvt, cvt_p};
 use bio::MemBio;
+use bn::BigNum;
 use error::ErrorStack;
 use nid::Nid;
 use string::OpensslString;
@@ -191,14 +192,24 @@ foreign_type_and_impl_send_sync! {
 }
 
 impl Asn1IntegerRef {
-    /// Returns value of ASN.1 integer, or -1 if there is an error, and 0 if the integer is Null.
-    ///
-    /// OpenSSL documentation at [`ASN1_INTEGER_get`].
-    ///
-    /// [`ASN1_INTEGER_get`]: https://www.openssl.org/docs/man1.1.0/crypto/ASN1_INTEGER_get.html
+    #[allow(missing_docs)]
+    #[deprecated(since = "0.10.6", note = "use to_bn instead")]
     pub fn get(&self) -> i64 {
         unsafe { ::ffi::ASN1_INTEGER_get(self.as_ptr()) as i64 }
     }
+
+    /// Converts the integer to a `BigNum`.
+    ///
+    /// This corresponds to [`ASN1_INTEGER_to_BN`].
+    ///
+    /// [`ASN1_INTEGER_to_BN`]: https://www.openssl.org/docs/man1.1.0/crypto/ASN1_INTEGER_get.html
+    pub fn to_bn(&self) -> Result<BigNum, ErrorStack> {
+        unsafe {
+            cvt_p(::ffi::ASN1_INTEGER_to_BN(self.as_ptr(), ptr::null_mut()))
+                .map(|p| BigNum::from_ptr(p))
+        }
+    }
+
     /// Sets the ASN.1 value to the value of a signed 32-bit integer, for larger numbers
     /// see [`bn`].
     ///
diff --git a/openssl/src/x509/mod.rs b/openssl/src/x509/mod.rs
index d4714f88b686d836695b5544ea708a505fe18732..011a2d96275c7989d84cdee203cedc3206e9dc57 100644
--- a/openssl/src/x509/mod.rs
+++ b/openssl/src/x509/mod.rs
@@ -523,6 +523,19 @@ impl X509Ref {
         }
     }
 
+    /// Returns this certificate's serial number.
+    ///
+    /// This corresponds to [`X509_get_serialNumber`].
+    ///
+    /// [`X509_get_serialNumber`]: https://www.openssl.org/docs/man1.1.0/crypto/X509_get_serialNumber.html
+    pub fn serial_number(&self) -> &Asn1IntegerRef {
+        unsafe {
+            let r = ffi::X509_get_serialNumber(self.as_ptr());
+            assert!(!r.is_null());
+            Asn1IntegerRef::from_ptr(r)
+        }
+    }
+
     to_pem! {
         /// Serializes the certificate into a PEM-encoded X509 structure.
         ///
diff --git a/openssl/src/x509/tests.rs b/openssl/src/x509/tests.rs
index ecc7f7de5eec63bffa2bfd937717cdf6395fea5f..fa8056ad6c7eea099e059c00954657e9c019bd1a 100644
--- a/openssl/src/x509/tests.rs
+++ b/openssl/src/x509/tests.rs
@@ -202,6 +202,7 @@ fn x509_builder() {
         .next()
         .unwrap();
     assert_eq!("foobar.com".as_bytes(), cn.data().as_slice());
+    assert_eq!(serial, x509.serial_number().to_bn().unwrap());
 }
 
 #[test]