Species of the genus Epilobium (Onagraceae) have been investigated for their activity against 5 alpha-reductase and aromatase, two enzymes which are involved in the aetiology of benign prostatic hyperplasia (BPH). Activity-guided fractionation has led to the identification of two macrocyclic ellagitannins, oenothein A (1) and oenothein B (2), as the main constituents responsible for the inhibition of the two enzymes. Quantitation of oenothein B in 10 different species of Epilobium has shown that amounts of up to 14% in the crude plant extracts are possible.