diff --git a/cmd/markdown/main.go b/cmd/markdown/main.go
index 22525ae..34d8e3c 100644
--- a/cmd/markdown/main.go
+++ b/cmd/markdown/main.go
@@ -5,13 +5,11 @@ import (
 	"flag"
 	"fmt"
 	"github.com/knieriem/markdown"
-	"io/ioutil"
+	"log"
 	"os"
 )
 
 func main() {
-	var b []byte
-
 	flag.Usage = func() {
 		fmt.Fprintf(os.Stderr, "Usage: %s [FILE]\n", os.Args[0])
 		flag.PrintDefaults()
@@ -21,10 +19,14 @@ func main() {
 	optDlists := flag.Bool("dlists", false, "support definitions lists")
 	flag.Parse()
 
+	r := os.Stdin
 	if flag.NArg() > 0 {
-		b, _ = ioutil.ReadFile(flag.Arg(0))
-	} else {
-		b, _ = ioutil.ReadAll(os.Stdin)
+		f, err := os.Open(flag.Arg(0))
+		if err != nil {
+			log.Fatal(err)
+		}
+		defer f.Close()
+		r = f
 	}
 
 	e := markdown.Extensions{
@@ -36,7 +38,7 @@ func main() {
 	startPProf()
 	defer stopPProf()
 
-	doc := markdown.Parse(string(b), e)
+	doc := markdown.Parse(r, e)
 	w := bufio.NewWriter(os.Stdout)
 	doc.WriteHtml(w)
 	w.Flush()
diff --git a/doc.go b/doc.go
index 79def4b..4a76f33 100644
--- a/doc.go
+++ b/doc.go
@@ -3,17 +3,16 @@ A translation of peg-markdown [1] into Go.
 
 Usage example:
 
+	package main
+
 	import (
-		md "markdown"
+		md "github.com/knieriem/markdown"
 		"os"
-		"io/ioutil"
 		"bufio"
 	)
 
 	func main() {
-		b, _ := ioutil.ReadAll(os.Stdin)
-
-		doc := md.Parse(string(b), md.Extensions{Smart: true})
+		doc := md.Parse(os.Stdin, md.Extensions{Smart: true})
 
 		w := bufio.NewWriter(os.Stdout)
 		doc.WriteHtml(w)	
diff --git a/markdown.go b/markdown.go
index ac7132c..0a29b4d 100644
--- a/markdown.go
+++ b/markdown.go
@@ -21,6 +21,7 @@ package markdown
 
 import (
 	"bytes"
+	"io"
 	"log"
 	"strings"
 )
@@ -35,7 +36,7 @@ type Extensions struct {
 }
 
 // Parse converts a Markdown document into a tree for later output processing.
-func Parse(text string, ext Extensions) *Doc {
+func Parse(r io.Reader, ext Extensions) *Doc {
 	d := new(Doc)
 	d.extension = ext
 
@@ -43,7 +44,7 @@ func Parse(text string, ext Extensions) *Doc {
 	d.parser.Doc = d
 	d.parser.Init()
 
-	s := preformat(text)
+	s := preformat(r)
 
 	d.parseRule(ruleReferences, s)
 	if ext.Notes {
@@ -109,31 +110,39 @@ const (
 /* preformat - allocate and copy text buffer while
  * performing tab expansion.
  */
-func preformat(text string) (s string) {
+func preformat(r io.Reader) (s string) {
 	charstotab := TABSTOP
-	i0 := 0
+	buf := make([]byte, 32768)
 
-	b := bytes.NewBuffer(make([]byte, 0, len(text)+256))
-	for i, _ := range text {
-		switch text[i] {
-		case '\t':
-			b.WriteString(text[i0:i])
-			for ; charstotab > 0; charstotab-- {
-				b.WriteByte(' ')
+	b := bytes.NewBuffer(make([]byte, 0, 32768))
+	for {
+		n, err := r.Read(buf)
+		if err != nil {
+			break
+		}
+		i0 := 0
+		for i := range buf[:n] {
+			switch buf[i] {
+			case '\t':
+				b.Write(buf[i0:i])
+				for ; charstotab > 0; charstotab-- {
+					b.WriteByte(' ')
+				}
+				i0 = i + 1
+			case '\n':
+				b.Write(buf[i0 : i+1])
+				i0 = i + 1
+				charstotab = TABSTOP
+			default:
+				charstotab--
+			}
+			if charstotab == 0 {
+				charstotab = TABSTOP
 			}
-			i0 = i + 1
-		case '\n':
-			b.WriteString(text[i0 : i+1])
-			i0 = i + 1
-			charstotab = TABSTOP
-		default:
-			charstotab--
-		}
-		if charstotab == 0 {
-			charstotab = TABSTOP
 		}
+		b.Write(buf[i0:n])
 	}
-	b.WriteString(text[i0:])
+
 	b.WriteString("\n\n")
 	return b.String()
 }