Serum prolactin levels at the time of confirmed diagnosis and of remission were determined prospectively in 36 patients (22 males, 14 females; mean age 54.2 [18-77] years), 12 with Hodgkin and 24 with non-Hodgkin lymphoma. Basal prolactin levels were elevated to a mean of 1900 (720- greater than 4000) mU/l in 12 patients. Eight of these had extranodal manifestations of lymphoma. At the time of remission the elevated prolactin levels had returned to normal in six patients. The cortisol concentrations of the 36 patients did not correlate with the prolactin levels so that stress-induced increase in secretion seems unlikely. These results point to prolactin possibly playing a role as a nonspecific tumour marker. In addition, lymphomas should be included in the differential diagnosis of hyperprolactinaemia.