To compare the characteristics of callers to a telephone helpline with those of smokers seeking assistance through a clinic and smokers in a general population sample. Data were gathered from callers to the Quitline in May and June 2000. Screening data were used for smokers attending the South London & Maudsley NHS Smokers' Clinic from May 2000 to April 2001. The results of the 2000/2001 General Household Survey (GHS) were used to estimate the characteristics of smokers in the general population. Callers to the Quitline (n=1162) surveyed during the summer of 2000, smokers attending the Maudsley Hospital Smokers' Clinic between May 2000 and April 2001 (n=1127), and smokers surveyed in the GHS between April 2000 and March 2001 (n=14,089) were compared on demographic characteristics, tobacco dependence and readiness to quit. Women and older smokers are more likely to seek help, but the Quitline attracts more calls from younger smokers. The age profile of helpline callers corresponds more closely to that of the general population of smokers than does the age profile of clinic attenders. Quitline callers are more dependent than the general population of smokers; clinic attenders are even more dependent. In comparison with the general population, the majority of Quitline callers have plans to quit smoking soon or have already stopped smoking. Telephone helplines reach a more representative sample of smokers in various states of readiness to quit, and provide a service for a population not reached by clinics.