Resistance of sunflower leaves to attack by Sclerotinia sclerotiorum was studied by infecting them, in the field, with agar disks containing Sclerotinia mycelium. Resistance levels were determined by the length of lesions after a given period. There were significant differences in reaction between both sunflower hybrids and inbred lines. Different Sclerotinia isolates gave the same classification of sunflower genotypes. The results of trials repeated in one year or different years were significantly correlated. The general combining ability variance/specific combining ability variance ratio was 1.35. Strict sense heritability was 0.61 + 0.03. The midparent-offspring correlation coefficient was significant, but the relation between per se values of the male parents used with the values of their hybrids was much closer than that for the female parents. Mean heterosis for resistance was 28.9%, compared with the midparent. The results of the leaf resistance test are frequently correlated with levels of resistance to root attack by Sclerotinia. Possible use of this test in breeding both directly for leaf resistance and indirectly for root resistance are discussed. H.P.L. Chromatography studies of the phenols present in healthy and infected leaves distinguished 19 compounds, all of the inhibitin type. There was a large increase in phenol content in leaves infected by Sclerotinia, for all genotypes. However, more especially in uninfected leaves, the contents of 3 chromatogram peaks, numbered 4, 6 and 9, showed a close relation with levels of Sclerotinia resistance. It is proposed that these compounds could be used as markers of certain types of resistance.