Objectives The aim of this study was to examine the risk of bladder cancer according to the trajectory pattern of amount of smoking among Korean young adult men. Methods Smoking status was assessed with a standardized questionnaire in the Korean Life Course Health Study (KLCHS). Trajectory analyses were performed among young adult men using seven repeated surveys of cigarette per day (CPD) every two years from 1992 to 2005. The occurrence of bladder cancer was tracked from 2006 to 2016. The Cox proportional models were used to calculate the hazard ratio (HR) (95% confidence interval) of smoking patterns on bladder cancer. Results The mean (standard deviation) age of the 161,069 participants was 34.0 (3.9) years, and 2,280,143 person-years (PY) were examined during the follow-up period of 14.2 (median 14.3) years. During this period, 263 new cases of bladder cancer occurred (11.5/100,000 PY). Among the six trajectory groups (low steady, lowering, rise and fall, high steady, rise and sharp fall, and very high steady), there was a higher risk of developing bladder cancer in the all the other groups compared to the low steady group. The highest risk group was the very high steady group, with HR 2.83 (95% CI 1.79–4.49). In addition, the risk of bladder cancer was 2.61 (95% CI 1.50–4.54) in the rise and sharp fall group. Conclusion The risk of bladder cancer did not show much difference according to trajectories, except for low steady group. Thus quitting smoking should be the priority to lower the risk of bladder cancer in smokers. Electronic supplementary material The online version of this article (10.1007/s10552-020-01335-8) contains supplementary material, which is available to authorized users.