The efficacy and tolerability of a long term treatment (21-53 months; mean, 36) with a new injectable form of bromocriptine (Parlodel LAR, Sandoz) was assessed in 13 patients (9 males and 4 females, aged 14-68 yr) with macroprolactinoma. Parlodel LAR was administered deeply im once monthly, with 50 mg as the first dose. Depending on the patient's tolerability to the drug and the PRL levels, the dose was individually progressively increased to 100 mg (2 patients), 150 mg (3 patients), or 250 mg (4 patients). Persistently normal PRL levels were recorded in 4 patients even after the first injection and in 5 other patients treated with higher doses of Parlodel LAR (2 patients with 100 mg/month; 3 patients with 150 mg/month). The remaining 4 patients who were treated with 250 mg/month had a marked reduction of PRL levels (72-94%), but did not reach normalization. Two patients treated with 150 mg/month maintained normoprolactinemia in spite of subsequent dose reduction of Parlodel LAR to 50-100 mg/month. In 1 patient PRL plasma concentrations remained within normal range for 3 months after the transitory discontinuation of Parlodel LAR at the end of the first year of therapy. Regular menses were resumed in 1 of 3, and galactorrhea disappeared in 2 of 3 women. All male patients had a return of libido and potency; gynecomastia disappeared in both male patients, and galactorrhea disappeared in 1 of 2 male patients. Visual fields improved in all 5 patients; complete normalization occurred in 2 of them. A consistent shrinkage of the macroadenoma (23-100%) at different times after therapy was shown by magnetic resonance imaging and/or computed tomography in 12 of 13 patients. Six patients reported mild/moderate side-effects (nausea, vomiting, orthostatic hypotension, or dizziness) within 24 h after the first injection. In 2 of these patients, mild side-effects persisted for 1-2 days after the first 3-6 injections, and in one patient, mild nausea was reported after each injection. In conclusion, in patients with macroprolactinoma, Parlodel LAR is an effective and well tolerated preparation of bromocriptine when administered once a month.