Aims: To compare gambling intensity between users of an online gambling service prompted to set a deposit limit and non-prompted customers, in the whole sample and among most active users based on total number of gambling days. Design: A randomized controlled trial with three prompted (N=1098/1110/1055) and one unprompted control condition (N=1065) and gambling activity of each participant followed for 90 days starting at account registration. Setting. A publicly governed online gambling operator from Finland. Interventions: The prompted participants received a one-time prompt to set a voluntary deposit limit of optional size either 1) at-registration, 2) before or 3) after they made their first deposit. Participants in the control condition were not prompted. Participants: All Finnish customers of the gambling service having registered an account during the study recruitment period with online slots as preferred gambling category during the 90 days following the registration. Measures: Gambling intensity was measured with aggregated net loss (NL), with proportions of limit-setters, sum of deposits and number of gambling days used as secondary outcomes. Findings: In the whole sample, the three intervention groups did not differ in either proportion of participants with positive NL (BF10prop=0.007) or size of positive net loss (BF10size=0.016), the results being similar for the most involved gamblers (BF10prop=0.030; BF10size=0.082). The pooled intervention group did not differ from the control group regarding proportion of gamblers with positive NL (OR(95%CI)=0.991(0.836–1.176); p=.921) or size of NL (B(95%CI) =-0.080(-0.229-0.069); p=.291), the results applying to the most involved gamblers (proportion positive NL: OR(95%CI)=0.834(0.492-1.412), p =.498; NL size: B(95%CI)=0.042(-0.359-0.442), p =.838). The three intervention groups had higher rates of limit-setters and NL was highest among participants who have increased/removed a deposit-limit. Conclusion: Prompting users of an online gambling service to set a voluntary deposit limit of optional size did not affect subsequent NL compared to unprompted participants.