First Majestic Silver (AG) had the highest return between 2001 and 2004 by a mid-cap US stock, returning 7,268.4%.

ASSET	YEARS	% RETURN
First Majestic Silver (AG)	2001-2004	7,268.44%
Urban Outfitters (URBN)	2001-2004	2,229.24%
Companhia Siderurgica Nacional ADR (SID)	2001-2004	1,793.69%
FirstCash (FCFS)	2001-2004	1,656.44%
Denison Mines (DNN)	2001-2004	1,490.24%
Dorman Products (DORM)	2001-2004	1,189.62%
Bank Ozk (OZK)	2001-2004	1,149.1%
Boyd Gaming (BYD)	2001-2004	1,095.29%
Penn National Gaming (PENN)	2001-2004	1,081.63%
Acadia Healthcare (ACHC)	2001-2004	1,007.69%
Rush Enterprises B (RUSHB)	2001-2004	977.03%
Hecla Mining (HL)	2001-2004	936.5%
Darling Ingredients (DAR)	2001-2004	896.57%
SSR Mining (SSRM)	2001-2004	789.2%
Middleby (MIDD)	2001-2004	758.12%
CarMax (KMX)	2001-2004	688.55%
Frontline (FRO)	2001-2004	686.27%
Light & Wonder (LNW)	2001-2004	684.21%
Option Care Health (OPCH)	2001-2004	682.77%
Thor Industries (THO)	2001-2004	660.49%
Merit Medical Systems (MMSI)	2001-2004	654.57%
VEON (VEON)	2001-2004	635.05%
Gerdau SA ADR (GGB)	2001-2004	600.73%
MSA Safety (MSA)	2001-2004	599.91%
Enerpac Tool (EPAC)	2001-2004	596.04%

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