The attraction of being able to use the internet for the recruitment of an epidemiologic cohort stems mainly from cost efficiency and convenience. The pregnancy planning study ('Snart-Gravid')-a prospective cohort study of Danish women planning a pregnancy-was conducted to evaluate the feasibility and cost efficiency of using internet-based recruitment and follow-up. Feasibility was assessed by examining patient accrual data over time, questionnaire-specific response rates and losses to follow-up. The relative cost efficiency was examined by comparing the study costs with those of an alternative non internet-based study approach. The target recruitment of 2,500 participants over 6 months was achieved using advertisements on a health-related website, supported by a coordinated media strategy at study initiation. Questionnaire cycle-specific response rates ranged from 87 to 90% over the 12-month follow-up. At 6 months, 87% of women had a known outcome or were still under follow-up; at 12 months the figure was 82%. The study cost of $400,000 ($160 per enrolled subject) compared favorably with the estimated cost to conduct the same study using a conventional non-internet based approach ($322 per subject). The gain in efficiency with the internet-based approach appeared to be even more substantial with longer follow-up and larger study sizes. The successful conduct of this pilot study suggests that the internet may be a useful tool to recruit and follow subjects in prospective cohort studies.