In animal and in vitro studies, several calcium antagonists have been shown to block apoptosis (programmed cell death), a natural cellular defence against cancer. On the basis of these studies, it has been hypothesised that calcium antagonists may function as cancer promoters and that they might cause cancer in humans. The association between the use of calcium antagonists and cancer has been addressed recently in 6 independent epidemiological studies. Four of these studies--2 cohort and 2 case-control--found a significantly higher risk of cancer among users of certain calcium antagonists as compared with either non-users or with users of other antihypertensive agents. The other 2 studies failed to find support for this association. All studies had limitations of varying types and significance. The emerging pattern from these investigations includes the following features: (i) the strength of the association appears to be dependent on daily dosage, ranging from no association in users of low dosages to a 2-fold (or possibly higher) increased risk in users of higher dosages; (ii) the time lag to the appearance of this association appears to be at least 2 to 3 years; (iii) an association with a higher risk of cancer has been found primarily for verapamil, while no such relationship has been reported for diltiazem; and (iv) no highly consistent associations have been shown with specific cancer sites or histological types. More data, preferably from long-term randomised clinical trials, are required before firm conclusions can be drawn.