Adults with acne represent an increasingly important population of acne sufferers referred for treatment. Acne, in these patients, is generally mild or moderate in severity but tends to be resistant to conventional antibiotic therapy. A study was carried out to assess the efficacy of intermittent moderate dose isotretinoin as a treatment for acne. Eighty consecutive patients, over the age of 25 years, referred with acne unresponsive to, or relapsing rapidly after three or more courses of conventional antibiotic therapy were recruited. Acne severity was assessed on the face, chest and back using the Leeds grading scale and the number of inflamed lesions was counted at the site showing the highest acne grade. The patients were 22 men and 58 women. The treatment regimen consisted of isotretinoin, 0.5 mg/kg per day for 1 week in every 4 week for a total period of 6 months. Seventy-five patients completed the study. The therapy was very well tolerated with mild cheilitis as the only side-effect. At the end of treatment, both total acne grade and lesion counts were significantly reduced (P < 0.0001). The acne had resolved in 68 (88%) patients. Twelve months after treatment, acne grades and inflamed lesion counts remained significantly improved (P < 0.0001) in the 68 patients who responded; however, 26 (39%) patients had relapsed. There was a significantly higher incidence of relapse in patients with predominantly truncal acne (P = 0.01). Patients who relapsed also had a significantly higher total acne grade, lesion count (P < 0.0001) and sebum excretion rate (P < 0.001) compared with those whose acne resolved. This study suggests that intermittent moderate dose isotretinoin may be a cost-effective alternative to full dose isotretinoin in a carefully selected group of adult patients with-acne. Selection criteria should include predominantly facial acne, total acne grade less than 1, inflamed lesion count less than 20 and sebum excretion rate less than 1.25 micrograms/cm2 per min.