We assess the impact of environmental externalities on portfolio decisions in a lab-inthe-field experiment on finance professionals and students. Subjects show pro-environmental preferences, with a strong asymmetry because of the sign of the externality. They are prone to accept lower return for positive environmental impact, but not to bear increased risk. Finance professionals are more pro-environmental than students, particularly regarding negative externalities, and less influenced by a ranking signal about environmental performance. Additional control tasks show that pro-social and pro-environmental preferences have much less influence on portfolio composition than market practices for finance professionals, but they are significant predictors for students.