The immunomodulatory macrolactams provide an alternative to glucocorticosteroids for the topical treatment of atopic dermatitis and other inflammatory dermatoses. Tacrolimus (FK506), as well as the newer ascomycin derivative ASM 981 (pimecrolimus), penetrate the inflamed epidermis and are suitable for topical therapy. Both substances inhibit the transcription of proinflammatory cytokine genes such as interleukin 2, which are dependent on the nuclear factor NF-AT. They block the catalytic function of calcineurin, which leads to the inhibition of the transport of the cytoplasmic component of NF-AT to the cell nucleus. Multicenter, randomized, double-blind clinical trials with topical formulations have shown the efficacy of both substances in moderate to severe atopic dermatitis. A review is presented of the biochemical and cell biologic properties, mode of action, pharmacokinetic data, side effects, results of the clinical trials, and further indications for tacrolimus and ASM 981, along with an overview of the related substances cyclosporine and sirolimus (rapamycin). (J Am Acad Dermatol 2001;45:736-43.)