LivaNova (LIVN) had the lowest return in May 1998 by a mid-cap US stock, returning -49.6%.

ASSET	MONTH	% RETURN
LivaNova (LIVN)	May 1998	-49.58%
Grupo Simec SAB de CV ADR (SIM)	May 1998	-32.86%
Companhia Paranaense de Energia Pref ADR (ELP)	May 1998	-30.57%
Sharplink Gaming (SBET)	May 1998	-27.5%
Sasol (SSL)	May 1998	-25.58%
CarMax (KMX)	May 1998	-24.39%
Vicor (VICR)	May 1998	-23.31%
Comstock Resources (CRK)	May 1998	-23.04%
Hecla Mining (HL)	May 1998	-22.33%
Seaboard (SEB)	May 1998	-21.91%
BBVA Banco Frances SA ADR (BBAR)	May 1998	-21.89%
Insight Enterprises (NSIT)	May 1998	-21.63%
InterDigital (IDCC)	May 1998	-21.43%
Argan (AGX)	May 1998	-21.43%
Kulicke and Soffa Industries (KLIC)	May 1998	-21.26%
Cognex (CGNX)	May 1998	-21.24%
Qorvo (QRVO)	May 1998	-20.54%
Companhia Siderurgica Nacional ADR (SID)	May 1998	-20.13%
Cheesecake Factory (CAKE)	May 1998	-20.05%
Caesars Entertainment (CZR)	May 1998	-20%
Patterson-UTI Energy (PTEN)	May 1998	-19.01%
Companhia Energetica de Minas Gerais CEMIG Pref ADR (CIG)	May 1998	-18.83%
Hub (HUBG)	May 1998	-18.08%
California Water Service (CWT)	May 1998	-17.82%
Alaska Air (ALK)	May 1998	-17.76%

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