In Sweden, individuals with dysplastic naevus syndrome (DNS-D2), a high risk group for malignant melanoma, are regularly screened and informed about self-examination and sun protection. During the summer of 1994, 54 out of 65 consecutive patients completed 1 month of daily self-recordings of sun-related behaviour. The diary report was compared with questionnaire responses obtained 6 months later concerning sun-related behaviour, both habitual and during the month of self-recording. The correspondence between the sun-related behaviour recorded in the diary and given in response to the questionnaire was fairly high, but 48% underestimated and 29% overestimated their actual number of sunbathing occasions in the questionnaire. Few patients indicated habitual high frequencies of sunbathing, although some of them recorded six or more occasions during 1 month in the diary. Those who recorded multiple sunburns reported the highest number of sunburns in the questionnaire. Patients who scored high on sunbed use also recorded high numbers of sunbathing occasions. Diaries should be used when detailed information about the magnitude of sun-related behaviour is essential, whereas questionnaires should be sufficient in studies aiming to differentiate between high and low frequencies of such behaviour.