The effect of a delay in starting 13-cis-retinoic acid treatment on the inhibition of urinary bladder carcinoma induced by N-butyl-N-(4-hydroxybutyl)nitrosamine was studied in male Fischer 344 rats. Animals received a total p.o. dose of either 1200, 1800 or 2400 mg N-butyl-N-(4-hydroxybutyl)nitrosamine over a period of six weeks. At either one, five, and nine weeks after the last N-butyl-N-(4-hydroxybutyl)nitrosamine intubation, animals were started on a diet supplemented with 13-cis-retinoic acid (240 mg/kg of laboratory chow) or continued on laboratory chow. Animals were killed at one year after the first carcinogen intubation for histological evaluation of the bladder. Feeding of 13-cis-retinoic acid reduced the incidence, average number, and severity of transitional cell carcinomas as well as hyperplasia and cellular atypia. Furthermore, even a nine-week delay in starting the retinoid feeding did not diminish the ability of 13-cis-retinoic acid to inhibit bladder carcinogenesis.