Background: For people with type 1 diabetes, there is no alternative to treatment with insulin. The major side effect of insulin is severe hypoglycaemia (SH), when the patient needs help or even becomes unconscious. Material: We have studied a geographic population of yearly 130-140 unselected type 1 diabetes patients aged 1-18 years during 1992-1999. They were intensively treated with 87-96% on 4-7 daily insulin doses, combined with active self-control, psychosocial support and problem-based education from onset. Average HbA1c was 6.5 with Mono-S standard (1.15% beow DCCT level). Methods: We evaluated use of a prospective patient questionnaire for continuous long-term registration of treatment and outcome data and analysed HbA1c, SH and other variables. Over years, 95-100% response rate was achieved. We used also temporary questionnaries. Results: We found SH with unconsciousness reported from on average 11% of patients yearly, SH without unconsciousness but needing assistance from on average 36% yearly and weak associations to HbA1c, such as reletive risk of SH 1.24 for yearly mean HbA1c <7.0% compared to ≥7.0% There was a seasonal variation in HbA1c (p=0.023) and incidence of SH. The strongest predictor for SH was SH during the previous year (r=9.38, p<0.0001). The impact from SH showed great variation, and 20-30% of events led to practical disturbancies for parents and/or other people. Hospital visits took place only at 5% and hospitalisations at 3% of events. Social activities for patients were cancelled after 10% of events. Increased worry for patients was reported after 8% of events, bad sleep after 7%. We estimated the average socio-economic cost for SH at EURO 239 per event of SH with unconsciousness, and EURO 63 per event of SH without unconsciousness but needing assistance. Mass-distributed self-study material (brochures and videos) aimed at the prevention of SH without compromising metabolic control reached high dissemination and was widely appreciated by patients. The material copy cost was only EURO 7 per patient. It also seems to have contributed to a decrease in SH with unconsciousness from yearly 13% of patients before to 9% after intervention (3-years average), but controlled studies are needed. Conclusions: We conclude that SH remains a very serious problem of multifactorial aethiology. It causes considerable discomfort and costs. Systematic patient education mgiht reduce the incidence. Interventions using mass-distribution of high quality self-study material such as videos and brochures seem to have a potential to be cost-effective. There is a great patient/consumer interest in high quality- and advanced information/education materials.