Human gingival fibroblasts from six patients who developed gingival hyperplasia as a result of nifedipine medication and five who did not were studied for the effects of calcium-channel blockers (nifedipine, diltiazem, verapamil and nicardipine) on cell proliferation, DNA synthesis and collagen synthesis. Phenytoin was used as a positive control. The fibroblasts from reactive patients gave trends toward better cell proliferation rates, DNA synthesis ([3H]-thymidine incorporation), and collagen synthesis ([3H]-proline incorporation) than those from non-reactive patients in the presence of 1 microM of calcium-channel blockers or phenytoin.