American Eagle Outfitters (AEO) had the highest return between 1997 and 1998 by a mid-cap US stock, returning 1,689.9%.

ASSET	YEARS	% RETURN
American Eagle Outfitters (AEO)	1997-1998	1,689.91%
Sirius XM (SIRI)	1997-1998	682.86%
CSG Systems International (CSGS)	1997-1998	426.66%
SkyWest (SKYW)	1997-1998	376.97%
Advanced Energy Industries (AEIS)	1997-1998	376.19%
Skyworks Solutions (SWKS)	1997-1998	357.13%
SEI Investments (SEIC)	1997-1998	349.71%
Abercrombie & Fitch (ANF)	1997-1998	325.56%
Semtech (SMTC)	1997-1998	318.97%
Insight Enterprises (NSIT)	1997-1998	308.81%
Plexus (PLXS)	1997-1998	304.48%
LivaNova (LIVN)	1997-1998	285.71%
Open Text (OTEX)	1997-1998	260.18%
Oshkosh (OSK)	1997-1998	247.51%
Lumen Technologies (LUMN)	1997-1998	234.6%
Kohl's (KSS)	1997-1998	212.06%
Molson Coors Brewing (TAP)	1997-1998	207.31%
Qorvo (QRVO)	1997-1998	199.2%
Buckle (BKE)	1997-1998	188.01%
Terex (TEX)	1997-1998	185.62%
Match (MTCH)	1997-1998	184.95%
IAC (IAC)	1997-1998	184.95%
Mohawk Industries (MHK)	1997-1998	183.57%
Stewart Information Services (STC)	1997-1998	182.67%
Colliers International Group Inc Bats (CIGI)	1997-1998	181.91%

Mid-cap stocks are treated as having a market cap between $2B and $10B.