OBJECTIVE: Better quality bystander first-aid could improve outcome rates for emergency victims significantly. In this case-control study, we hypothesised that expert knowledge presented step-by-step to untrained helpers using a personal digital assistant (PDA), would improve the quality of bystanders basic life support. METHOD: We confronted 101 lay-helpers with two standard emergency situations. (1) An unconscious trauma victim with severe bleeding. (2) Cardiopulmonary resuscitation (CPR). Performance was assessed using an Objective Structured Clinical Examination (OSCE). One group was supported by a PDA providing visual and audio instructions, whereas the control group acted only with their current knowledge. The expert system was programmed in HTML-code and displayed on the PDA's Internet browser. RESULTS: The maximum score obtainable was 24 points corresponding to optimal treatment. The control group without the PDA reached 14.8+/-3.5 (mean value+/-standard deviation), whereas the PDA supported group scored significantly higher (21.9+/-2.7, p<0.01). The difference in performance was measurable in all criteria tested and particularly notable in the items: placing in recovery position, airway management and quality of CPR. CONCLUSION: The PDA based expert system increased the performance of untrained helpers supplying emergency care significantly. Since Internet compatible mobile devices have become widely available, a significant quality improvement in bystander first-aid seems possible.