The therapeutic efficacy of L-deprenyl (10 mg daily) as an adjunct to low-dose bromocriptine monotherapy (up to 25 mg daily) in patients with early Parkinson's disease (PD) was evaluated in a double-blind placebo-controlled short-term study (11 patients) and subsequently in a long-term prospective open follow-up (21 patients) until L-dopa was required, over a 4-year period. The combined regimen of bromocriptine plus L-deprenyl produced a mildly significant improvement, as shown by the majority of clinical rating scales used after 6 weeks of sustained treatment (as compared to bromocriptine alone and bromocriptine plus placebo). In the prospective long-term study, a stabilization of the clinical status was observed until 12 months of sustained treatment, whereas after that, a gradual worsening of the scores on all motor rating scales occurred. However, at 24 months, fewer than one third of the patients had required L-dopa, a proportion comparatively smaller than that reported in the literature with bromocriptine alone. This finding could be related to the persistence of initial symptomatic effect of L-deprenyl, but a slowing action on the course of the disease process exerted by the monoamine oxidase typeB (MAO-B) inhibitor cannot be ruled out.