OBJECTIVE: The recent advances in psycho-neuro-endocrino-immunology have demonstrated the existence of several endogenous neuroendocrine substances, capable of affecting both tumor growth and host anticancer immune defenses. The pineal gland would represent one of the most important organs releasing antiproliferative and immunostimulating substances, the most known of them is melatonin (MLT). However, MLT would not be the only pineal indole provided by antitumor activity. Other pineal indoles, namely 5-methoxytryptamine (5-MTT), would play antitumor effects, by either inhibiting cancer cell proliferation or stimulating the anticancer immunity. Preliminary data have shown that MLT may deserve antitumor activity in the treatment of human neoplasms, whereas at present there are no clear data about 5-MTT. In an attempt to obtain some preliminary data about the anticancer properties of 5-MTT in humans, we have evaluated the efficacy of MLT plus 5-MTT in untreatable advanced cancer patients progressing on MLT alone. METHODS: The study included 73 untreatable advanced solid tumor patients, who had progressed after two months of MLT therapy alone. According to tumor histotype, patients were randomized to receive MLT alone (20 mg/day orally in the evening) or MLT plus 5-MTT (1 mg at noon orally), every day for at least two months. The clinical response was evaluated according to WHO criteria. RESULTS: A partial response (PR) occurred in two patients treated with MLT + 5-MTT and in none of the patients receiving MLT alone. A stable disease (SD) was achieved in only 2/37 patients on MLT therapy alone, and in 8/36 patients receiving MLT plus 5-MTT. Therefore, the percent of non-progressing patients (SD + PR) obtained with MLT plus 5-MTT was significantly higher than that obtained with MLT alone. Moreover, the relief of asthenia and depressant symptoms was significantly higher in patients concomitantly treated with 5-MTT. DISCUSSION: This preliminary study would suggest that the concomitant administration of the less known pineal indole 5-MTT, also provided by antiproliferative and immunomodulating effects, may further amplify the oncostatic activity of the pineal hormone MLT in the palliative and curative therapy of advanced untreatable human solid neoplasms.