BACKGROUND: Diaries are used in sexual behaviour research to reduce recall bias. Diary collection via mobile phone text messaging (SMS) has not been trialled previously in sexual behaviour research. This randomised controlled trial compared SMS, paper and online diaries on response rate, timeliness, completeness of data and acceptability. The correlation between behaviour reported in all three types of diaries and data collected in a retrospective questionnaire was also determined. METHODS: Participants were recruited by telephone and randomised into one of three groups. They completed weekly sexual behaviour diaries for 3 months by SMS, online or paper (by post). An online survey was conducted at the end of 3 months to compare retrospective reports to the diaries and assess opinions on the diary collection method. RESULTS: 72 participants were enrolled in the study, 24 in each group. Online diaries were more likely to be submitted late than SMS diaries (p<0.001). 3.9% of SMS diaries, 3.1% of paper diaries and 0.5% of online diaries were incomplete (p=0.001). Online data collection was the preferred mode for 51%. 65 participants completed the end point retrospective questionnaire. The correlation between the diary and questionnaire on sexual risk classification was substantial (kappa=0.74) regardless of diary mode. CONCLUSIONS: SMS is a convenient and timely method of collecting brief behavioural data, but online data collection was preferable to most participants and more likely to be complete. Data collected in retrospective sexual behaviour questionnaires were found to agree substantially with data collected through weekly self-reported diaries.