B737-500
Flight Crew Last 90 Days 358; Flight Crew Type 5700
Communication Breakdown
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance
Human Factors
I was the pilot flying. We were cleared direct to DAG and to FL180. Passing 17;300 FT we set the altimeters to 29.92 in anticipation of leveling at FL180. Shortly after this I heard ATC clear us to FL190. I changed the altitude in the autopilot window and verbalized the altitude assignment. The Captain acknowledged FL190. As we climbed through FL180; the Captain said we were only cleared to FL180. I disconnected the autopilot and started to level the aircraft passing FL183. ATC; during this time; asked if we were leveling at FL180. The Captain told him we had overshot the altitude and were descending. The aircraft topped out at about FL185 and I expedited a descent back to FL180. ATC gave avoidance vectors to another aircraft and turned us right to 090. At no time were any TCAS alerts given. After he verified we were at FL180; he cleared us back to DAG. The rest of the flight was uneventful. I obviously misheard ATC instructions. Thinking back; he may have told us of traffic at FL190 and somehow I interpreted it as a climb to FL190. The Captain automatically repeated my FL190 call without thinking; as the Captain knew we were only cleared to FL180. Attention to detail on both of our parts needs to be emphasized. I give the ATC Controller a lot of credit for seeing the situation and quickly vectoring aircraft to avoid a conflict.
Air Carrier flight crew climbed above ATC assigned altitude. The Controllers referenced the altitude of traffic which may have contributed to the misunderstanding.