Fifty women with refractory obesity received phentermine resinate. Seven were withdrawn because of side-effects: three developed severe headaches, one each hypertension, depressive symptoms, breathlessness and palpitations with irritability. The mean weight loss in the 34 who completed the 20-week study was 6.4 kg. Nine lost 10 kg or more. Sustained appetite suppression was related to weight loss. Plasma phentermine concentrations did not correlate with the severity of the obesity problem, the degree of subjective anorexia or with weight loss. Poor initial response to standard dosage of phentermine is unlikely to improve with higher dosage. The individual's response to phentermine is unpredictable and appears to relate to factors other than the plasma drug concentration.