AN NMAC OF 200 FT VERT SEPARATION WAS RPTED BTWN A B747-400 ON LIFTOFF FROM RWY 14R AND AN ATR42 ON LIFTOFF FROM RWY 22L 100 FT AGL AT ORD; IL.

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CAPT WAS PF. 46 PAX AND 3 CREW. TAXIED TO RWY 22L AT ORD. ATIS INFO WAS CURRENT. WINDS 270 DEGS AT 7 KTS; VISIBILITY 10 MI; CLOUDS FEW AT 2800 FT; 28 DEGS DEWPOINT 22 DEGS; ALTIMETER SETTING 29.73. ARR RWY 14R; RWY 22R AND RWY 22L. DEP RWY 22L; RWY 27L AND RWY 14L. WE WERE CLRED INTO POS AND HOLD ON RWY 22L (TWR FREQ 132.7). THERE WERE CITY VEHICLES ON THE RWY; LOOKING FOR FOD THAT HAD BEEN RPTED BY A PREVIOUS ACFT. ONCE THE VEHICLES WERE CLR OF THE RWY; WE WERE CLRED FOR TKOF AND TO TURN TO A HDG OF 180 DEGS. MY FO REPEATED BACK THE CLRNC. WE ACCELERATED DOWN THE RWY AND AFTER V1; VR AT LIFTOFF WE SAW A B747 OFF OUR R SIDE; TAKING OFF OF RWY 14R. TWR CALLED THE TFC AT THAT TIME. WE LEVELED OFF ABOUT 100 FT AGL WITH NO ZERO G'S. THE B747 FLEW OVERHEAD ABOUT 200 FT ABOVE US. ONCE THE B747 PASSED AND WE WERE CLR; WE CONTINUED THE CLB AND FOLLOWED THE COMMAND BARS ON THE EADI. THE FLT CONTINUED WITH NO FURTHER PROBS AND THERE WERE NO INJURIES. I DON'T BELIEVE THE B747 ACFT WAS ON THE SAME TWR FREQ AS WE WERE. I OR MY FO DID NOT HEAR A CLRNC FOR TKOF ON RWY 14R. WE DID NOT KNOW THEY WERE DEPARTING RWY 14R. AN NMAC RPT WAS FILED WITH ORD CTL TWR. SUPPLEMENTAL INFO FROM ACN 481921: WE DEPARTED RWY 14L ON FLT ORD-HKG. WE WERE NEAR MAX GROSS WT (APPROX 865000 LBS). WE DID NOT SEE ANYTHING; BUT UPON RETURN TO ORD 6 DAYS LATER I WAS TOLD THAT AN ATR WAS TAKING OFF ON RWY 22L AT THE SAME TIME AND HAD TO LEVEL OFF AT 100 FT TO AVOID US. THIS WAS THE ONLY INFO THAT WAS RPTED TO US.