Melasma is a common disorder of cutaneous hyperpigmentation predominantly affecting the faces of women. Little is known about the aetiology of melasma, and treatment is frequently disappointing. Topical tretinoin is of benefit in treating other forms of hyperpigmentation, for example liver spots, and we therefore investigated its effectiveness in melasma. Thirty-eight women completed a randomized, vehicle-controlled study, in which they applied 0.1% tretinoin (n = 19) or vehicle cream (n = 19) once daily to the face for 40 weeks. At the end of treatment 13 (68%) of 19 tretinoin-treated patients were clinically rated as improved or much improved, compared with 1 (5%) of 19 in the vehicle group (P = 0.0006). Significant improvement first occurred after 24 weeks of tretinoin treatment. Colorimetry (an objective measure of skin colour) demonstrated a 0.9 unit lightening of tretinoin-treated melasma and a 0.3 unit darkening with vehicle (P = 0.01); these results correlated with clinical lightening (r = 0.55, P = 0.0005). Histologically, epidermal pigment was reduced 36% following tretinoin treatment, compared with a 50% increase with vehicle (P = 0.002). Reduction in epidermal pigment also correlated with clinical lightening (r = -0.41, P = 0.01). Moderate cutaneous side-effects of erythema and desquamation occurred in 88% of tretinoin-treated and 29% of vehicle-treated patients. Topical 0.1% tretinoin produces significant clinical improvement of melasma, mainly due to reduction in epidermal pigment, but improvement is slow.