The present invention pertains to the use of certain 3,4-diarylpyazoles of formula (I), both in vitro and in vivo, to inhibit heat shock protein 90 (HSP90), and in the treatment of conditions mediated by HSP90, including, for example, cancer; wherein: Ar.sup.3 is independently: a C.sub.5-20aryl group, and is optionally substituted; Ar.sup.4 is independently: a C.sub.5-20aryl group, and is optionally substituted; R.sup.5 is independently: hydrogen; halo; hydroxyl; ether; formyl; acyl; carboxy; ester; acyloxy; oxycarbonyloxy; amido; acylamido; aminocarbonyloxy; tetrazolyl; amino; nitro; cyano; azido; sulfhydryl; thioether; sulfonamide; C.sub.1-7alkyl; C.sub.3-20heterocycyl; or C.sub.5-20aryl; RN is independently: --H; C.sub.1-7alkyl; C.sub.3-20heterocycyl; or, C.sub.5-20aryl; and pharmaceutically acceptable salts, solvates, amides, esters, ethers, chemically protected forms, and prodrugs thereof. The present invention also pertains to such compounds, pharmaceutical compositions comprising such compounds, such compounds for medical use, such compounds use in the treatment of conditions mediated by HSP90, including, for example, cancer, and use of such compounds in the preparation of medicaments for such treatments.