Background: Calcipotriene is often used with UVB or PUVA, but interactions between UV radiation and calcipotriene have not been examined extensively. Objective: Our purpose was to examine interactions between calcipotriene and UV light. Methods: Minimal erythema doses (MEDs) were determined with UVB and immediate pigment darkening was measured for UVA. The effect of calcipotriene ointment applied before phototesting was examined. Thick and thin applications of calcipotriene were compared. Calcipotriene ointment was applied to a small area on the skin before phototherapy. Patients received either UVB, PUVA, UVA, or no phototherapy. After phototherapy, the ointment was collected and assayed by reverse-phase, high-performance liquid chromatography. Results: MEDs for UVB and immediate pigment darkening for UVA were unaffected by calcipotriene. Thick application of calcipotriene, however, increased the MED. UVA caused substantial reductions in the concentration of detectable calcipotriene. Conclusion: When used in conjunction with PUVA, calcipotriene should be applied after exposure to UVA. (J Am Acad Dermatol 1997; 37:93–5.)