help with a sed (i think) command

I've got a file that looks like: -

001
abcde
002
fghij
003
klmno
004 ......

and i want to use a sed command that will change the file to look like: -

001 abcde
002 fghij
003 klmno
004 ......

i've tried things like s/00\n//g but they don't seem to work. Any ideas?
Thanks,

hi,
maybe that can help :

$ more my_script
#!/usr/bin/ksh

cat $1 | while read LINE
do
case $LINE in
*[0-9])
echo $LINE>>/home/number
;;
*)
echo $LINE>>/home/alpha
;;
esac
done

sdiff /home/number /home/alpha | sed s/\|// > /home/ result

$ ./myscript your_file
$ more result
001 abcde
002 fghij
003 klmno
004 ......

is this helping?

A truly sed-only solution:

#! /usr/bin/sed -f
#
#  this script will join every other line
N       ;# suck in a second line
s/\n/ / ;# change new-line to space

or a more conventional script:

#! /usr/bin/ksh
sed -e 'N;s/\n/ /'
exit 0