A double-blind, parallel-group multicentre study has been done to compare the antihypertensive properties, effects on serum lipoproteins and adverse effect profiles of diltiazem and metoprolol given as monotherapy to primary hypertensive patients. 128 patients were included from 10 participating centers. Following a placebo wash-out period of 5 weeks, patients were randomized either to diltiazem or metoprolol treatment according to a forced titration regimen. Each dose was given for a 4-week period in a stepwise regimen. A total of 119 patients, 59 and 60 in the two groups, completed the study. Supine and standing BPs were reduced in a similar, dose-dependent fashion by diltiazem and metoprolol. In the former supine BP fell from 161/101 to 151/91 mmHg at the highest dose level. In the latter patients, supine BP at the highest dose level was reduced from 161/102 to 155/94 mmHg. Target pressures (DBP less than or equal to 90 mmHg and/or DBP reduction of greater than or equal to 10%) were reached in 63% and 48% of the patients, respectively. HDL-cholesterol was increased in diltiazem-treated patients and decreased in those on metoprolol. Otherwise, serum lipoproteins did not differ significantly between treatments. The incidence and severity of dose-dependent adverse effects did not differ significantly between treatments, although moderate to distressing side effects were reported more commonly by metoprolol-treated patients. Ankle oedema and breathlessness tended to be more common on diltiazem therapy, while tiredness, increased sweating and sleep disturbances appeared to be experienced more frequently by metoprolol-treated patients.(ABSTRACT TRUNCATED AT 250 WORDS)