Tazarotene in combination with phototherapy is being used clinically for the treatment of plaque psoriasis. This study investigates the dose of UVB light required to induce minimal erythema and the dose of UVA light required to induce immediate pigment darkening, with and without pretreatment with tazarotene 0.1% gel. The photostability of tazarotene is also assessed. Pretreatment with tazarotene 0.1% gel 3 times per week for 2 weeks before phototherapy significantly reduced the mean minimal erythema dose (MED) for UVB from 56.25 to 42.50 mJ/cm² (P < .01), and significantly reduced the mean UVA exposure required to induce immediate pigment darkening from 20.18 to 18.50 J/cm² (P < .05). A thin application of tazarotene gel immediately before phototherapy had no significant effect on the mean MED for UVB, whereas a thick application of the gel increased the MED slightly, from 56.25 to 62.50 mJ/cm² (P = .1). Tazarotene remained chemically stable when used in conjunction with UVB or UVA phototherapy. To reduce the patient’s potential to burn or tan, we recommend initiating UVB phototherapy at 50% to 75% of the MED when it is used in combination with tazarotene. We also recommend initiating PUVA therapy at slightly lower doses than usual. Lower total doses of UVA or UVB may be needed when patients with psoriasis are treated concomitantly with tazarotene. (J Am Acad Dermatol 1999;41:927-30.)