Hydroxyurea is an anti-tumour agent most commonly used to treat chronic myeloproliferative disorders in doses up to 4 g per day. Dermatological adverse effects reported so far have been observed predominantly in these patients. As we are treating selected psoriasis patients with low dose hydroxyurea we attempted to define the spectrum and chronology of dermatological adverse effects in this group of patients prospectively. Of the 29 evaluable patients, 19 (65.5%) developed a mucocutaneous adverse reaction after a mean duration of 6.4 weeks of treatment. Pigmentation of nails, skin or mucosa was the most common observation and was seen in 17 (58.6%) patients. Other less common findings were xerosis, diffuse alopecia, oedema of the legs, oral ulcers and actinic psoriasis. Adverse effects subsided in 11 (57.9%) patients during a mean follow up of 18 weeks. Three hitherto unreported side-effects - scleral pigmentation, acquired ichthyosis and pigmentation of lunula of the nails - were noted. This first study of dermatological adverse effects of hydroxyurea therapy on Asian psoriatic patients reveals several new findings. Pigmentation of skin, nails and mucosa appears to be very common and occurs early. Serious dermatological side-effects probably do not occur with low dose (up to 1.5 g per day) hydroxyurea in patients with psoriasis.