AT CVG FLC AND GND MECHS LEFT THE NOSE GEAR PIN INSTALLED. FLC IS UNABLE TO RETRACT NOSE GEAR WHEN AIRBORNE. ACFT RETURN LAND.

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DISPATCHED AS FLT XX CVG TO TOL; ALL CHKLISTS WERE COMPLETED AS REQUIRED TO PUSHBACK. AT DEP TIME WE WERE PUSHED BACK AND SIGNALED TO SET BRAKES. BOTH GND PERSONS REMOVED TOW BAR AND REAPPEARED. THEY INDICATED THEY COULD NOT FIND THE NOSE GEAR SAFETY PIN. I (CAPT) VISUALLY CHKED THAT ALL SAFETY PINS WERE ABOARD AND IN THE PIN HOLDER. I SIGNALED THE GND CREW TO AGAIN CHK FOR THE NOSE SAFETY PIN. THEY GAVE AN 'I DON'T KNOW/NOT THERE' SHRUG. WE TAXIED TO RWY 27 FOR SUBSEQUENT TKOF. AFTER TKOF WITH A POSITIVE RATE OF CLB; I ASKED FOR 'GEAR UP.' A FEW SECONDS LATER AT ABOUT 200-400 FT AGL; A 'GEAR DISAGREEMENT' AURAL WAS HEARD AND THEN VERIFIED BY A RED (UNSAFE) NOSE GEAR INDICATOR WHILE BOTH MAIN MOUNTS WERE UP (GREEN). I ENGAGED THE AUTOPLT AND REQUESTED THE 'GEAR DISAGREEMENT' CHKLIST. PER THE CHKLIST; AIRSPD WAS CHKED TO BE BELOW 250 KIAS (IT WAS 180 KIAS); VERIFY HYD PUMP 3B ON (ALL 4 ELECTRICAL PUMPS WERE ON AND RUNNING AS FLAPS WERE STILL AT 20 DEGS). THE LNDG GEAR LEVER WAS LOWERED (DOWN). ALL GEAR THEN INDICATED SAFE (DOWN). FO HANDLED CHKLISTS AND I HANDLED ATC COM AND REQUESTED TO RETURN TO ARPT FOR LNDG. AN EMER WAS NOT DECLARED; HOWEVER; NATURE OF PROB WAS RELAYED TO ATC. APCH/LNDG AND RETURN TO A GATE WAS UNEVENTFUL. MAINT PERSONNEL MET ACFT AT GATE FOR INSPECTION. INSPECTION REVEALED A SAFETY PIN WAS STILL INSTALLED WITH A LONG RED FLAG ATTACHED. PAX WERE INFORMED OF FINDINGS AND OF OUR INTENT TO CONTINUE ON TO TOL. WE WERE REDISPATCHED AND REMAINDER OF FLT WAS UNEVENTFUL. I'M NOT SURE HOW TO REMEDY THIS PROB. THE AIR CREW HAS TO PUT SOME FAITH IN GND CREWS. POSSIBLY THE NOSE DOORS SHOULD BE LEFT IN THE OPEN POS DURING PUSHBACK. PIN REMOVED AND THEN DOOR CLOSED AT COMPLETION OF PROC.