Mazindol is chemically unrelated to the phenethylamines and has not shown the side-effects or abuse potential of the amphetamine anorectics. To further define its potential for causing weight loss, a six-week double-blind placebo controlled study was undertaken in four centres. A common protocol was used except in one centre, behavioural modification also was employed, whereas in the other centres, no additional measures were used to cause weight loss. Two hundred and forty-five obese patients were assigned randomly to two mazindol groups and one placebo group in each centre. Ninety-eight and forty patients receiving mazindol and placebo respectively completed the protocol. The conclusions were: (a) no significant clinical or laboratory abnormalities occurred from mazindol therapy, (b) the placebo therapy patients did not lose weight without behavioural modification, (c) the placebo therapy group had a higher drop-out rate compared to the mazindol therapy group attributable to the patients' dissatisfaction with failure to lose weight, (d) mazindol therapy without behavioural modification and behavioural modification alone both resulted in a statistically significant mean weight loss of 1 pound/patient/week and (e) mazindol plus behavioural modification resulted in a greater mean weight loss of 1/2 pound/patient/week than with behavioural modification alone. Hence, mazindol is of value in the initial therapy of obesity.