In an investigation of relapse patterns, 5541 cases of Plasmodium vivax malaria, from four major industrial complexes, each received at least one, 5-day course of primaquine (at 15 mg/day). Any subject relapsing was retreated with the same course. Overall, 511 (9.2%) of the P. vivax cases relapsed after the first course and 99 (1.78%), 25 (0.45) and three (0.05%) cases relapsed two, three and four times, respectively. Most cases of relapse occurred within 1 year of treatment. Clearly, a 5-day primaquine regimen is inadequate to control relapses among P. vivax cases and there is therefore an urgent need to review the treatment strategy. It may now be appropriate to implement the 14-day regimen recommended by the World Health Organization, although this is much less feasible under field conditions.