The overall evidence of an association between fluid intake and bladder cancer is not entirely consistent. We examined the fluid intake-bladder cancer relationship in the Los Angeles bladder cancer case-control study. A total of 1,586 cases and their age-, sex-, and race-matched neighborhood controls were interviewed in-person from 1987 to 1999. Information on total fluid intake was derived from the consumption of specific fluids including water, coffee, tea, alcohol, milk, juice, hot chocolate and soda. Total fluid intake was not associated with bladder cancer. Daily water intake was associated with a slight decrease in bladder cancer risk, with the protection more pronounced among women (p for trend = 0.039) than among men (p for trend = 0.62). Compared to drinking <1 glass of water per day, drinking > or =6 glasses/day was associated with 0.91 (95% confidence interval, 0.67-1.22) times the risk of bladder cancer among all subjects, 0.94 (0.67-1.32) times the risk among men, and 0.69 (0.36-1.33) times the risk among women. The water intake-bladder cancer association also seemed to be modified by daytime urination frequency with significant inverse association among subjects who urinated > or =6 times/day (p for trend = 0.015), but not among those who urinated less frequently. Similarly, the protection from water intake was confined to women who did not experience nocturia and to men who did. Results from our study suggest that water intake may be associated with a slight reduction in bladder cancer risk.