The use of lithium, in the treatment and prevention of affective disorders, is considered to be one of the most important break through in modern Psychopharmacology. Side effects are frequently associated with its short and long term use. One of those side-effects, which is common after chronic lithium salt treatment, is a considerable increase in body weight. This side-effect appears to be an important factor in prompting some patients, particularly women, to suspend treatment. In this study, confirming our previous work with an animal model, we found a statistically greater weight increase in women treated chronically with lithium than in men. Our results were apparently not explained by abnormalities in thyroid function. The literature describes those thyroid abnormalities, due to lithium, as being more frequent in women than in men. In an animal model, we now found that higher plasma lithium levels are produced in male, when compared to female rats, when this drug is administered for long periods. If the some tendency is present in men, we could expect a greater probability of toxicity in men than in women. Thus, a lower weigh increase in men. Such could be a possible explanation for our clinical results.