Our aim was to compare the degree of patch test positivity to acetone and aqueous extracts of Parthenium hysterophorus in patients with airborne contact dermatitis. We performed patch testing with the Indian standard series (which includes aqueous extracts of parthenium, xanthium and chrysanthemum), and with 1 : 100 and 1 : 200 dilutions of an acetone extract of parthenium in 72 patients with airborne contact dermatitis. All patients showed contact sensitivity to the 1 : 100 dilution and 67 patients had positive allergic reactions to the 1 : 200 dilution of the acetone extract, whereas only 45 patients showed a positive reaction to the aqueous extract of P. hysterophorus. Our results confirm that parthenium allergens are more soluble in acetone than in water, and that the acetone extract is significantly better in detecting contact sensitivity to parthenium in patients with suspected plant dermatitis. Hence, the acetone extract is recommended for routine patch testing.