The International Agency for Research on Cancer (IARC) convened a working Group of experts in March 1999 to evaluate the cancer preventive potential of nine retinoids and to compile the fourth volume of the IARC Handbooks of Cancer Prevention. The handbook provides a comprehensive review of the relevant information in the published scientific literature through March 1999 on the potential role of all-trans-retinoic acid, 13-cis-retinoic acid, 9-cis-retinoic acid, all-trans N-(4-hydroxyphenyl)retinamide, etretinate, acitretin, N-ethylretinamide, targretin and LGD 1550 in cancer prevention. Of these, the data suggest that all-trans-retinoic acid, 13-cis-retinoic acid and N-ethylretinamide are not suitable for chemoprevention of cancer in humans either because they are too toxic, may enhance cancer occurrence or are ineffective. In contrast, 9-cis-retinoic acid, etretinate and acitretin show some promise, but more data are required, while all- trans N-(4-hydroxyphenyl)retinamide is quite promising. Targretin and LGD 1550 are of interest, based on theoretical grounds, but there are no significant human and little experimental data as yet.