This article documents a link between trading and diversification by using detailed trading records from a Swedish discount broker matched with individual tax records. Diversification is measured by the investors’ stake size, defined as the fraction of their risky financial wealth invested in individual stocks through the broker under study. High-stake investors have concentrated portfolios, trade more, and achieve lower trading performance. They share several features with those who trade excessively, namely lower income, wealth, age, and education, suggesting that they lack investment expertise. The results directly imply that trading losses in the cross-section are mainly borne by those who can least afford them.