An hyperprolactinemia, with basal serum prolactin levels ranging from 41 to 135 ng/ml was found to be coincidentally associated with psychosocial dwarfism in a 11 year-old boy. Sephadex G 100 exclusion chromatography showed that the predominating form of immunoreactive prolactin levels ranging from 41 to 135 ng/ml was found to be weight, differing from the regular occurrence of a 22 kilodalton major variant. Prolactin levels increased under TRH (increments between 29 and 76%) but were not blunted by bromocriptine at a dose of 2.5 mg/day. This so-called macroprolactinemia syndrome should be searched for whenever a discrepancy is noted between clinical symptoms and blood prolactin levels.