This paper reevaluates the cross-sectional effect of institutional ownership on idiosyncratic volatility by conditioning on institutions' investment horizon. Prior literature establishes a positive link between growing institutional ownership and idiosyncratic volatility. However, this effect may vary depending on the type of institutional ownership. We document that short-term (long-term) institutional ownership is positively (negatively) linked to idiosyncratic volatility in the cross section. These opposite effects persist after controlling for institutional preferences and information-based trading and remain qualitatively unchanged after controlling for endogeneity. This suggests that short-term (long-term) institutions exhibit higher (lower) trading activity, which increases (decreases) idiosyncratic volatility.