OBJECTIVE: Research in general practice has grown considerably over the past decades, but many projects face problems when recruiting patients. Lasagna's Law states that medical investigators overestimate the number of patients available for a research study. We aimed to assess factors related to success or failure of recruitment in general practice research. STUDY DESIGN AND SETTING: Survey among investigators involved in primary care research in The Netherlands. Face-to-face interviews were held with investigators of 78 projects, assessing study design and fieldwork characteristics as well as success of patient recruitment. RESULTS: Studies that focused on prevalent cases were more successful than studies that required incident cases. Studies in which the general practitioner (GP) had to be alert during consultations were less successful. When the GP or practice assistant was the first to inform the patient about the study, patient recruitment was less successful than when the patient received a letter by mail. There was a strong association among these three factors. CONCLUSION: Lasagna's Law also holds in Dutch primary care research: many studies face recruitment problems. Awareness of study characteristics affecting participation of GPs and patients may help investigators to improve their study design.