27 patients (12 women and 15 men) with hyperprolactinemia were treated with the dopamine agonists lisuride (Dopergin) and bromocriptine on a cross-over basis for 3-6 months. By this treatment, plasma prolactin levels were reduced by 83 and 87%, and normalization was achieved in 13 patients in the lisuride group (average dosage 1 mg/d) and in 15 patients in the bromocriptine group (average dosage 10 mg/d). Side effects, consisting of orthostatic hypotension, vomiting and nausea, occurred in 11 patients treated with lisuride and 13 patients treated with bromocriptine, and 2 patients in the lisuride group and one in the bromocriptine group stopped the medication for this reason. There was a good correlation in the doses of both drugs used. There was a tendency for the prolactin-lowering effect of lisuride to last longer upon cessation of therapy which might be related to the higher receptor affinity of both drugs. In general, however, the present results indicate that bromocriptine and lisuride are comparable in efficacy and side effects, but that individual patients respond better to one or the other drug.