The relationship between risk of cutaneous malignant melanoma and total body and site-specific naevus counts and other host factors was investigated in a Caucasian population aged 15-84 years in New South Wales, Australia. The study sample comprised 244 cases with melanoma diagnosed in 1989-1993, and 276 controls. The strongest relationship was with total body naevus count. Risk of melanoma was raised 12 times in those with more than 100 naevi compared with those with less than 10. There were also strong risks, with odds ratios of 5 or more, associated with having multiple atypical naevi, multiple large naevi, high naevus counts in sun-exposed or sun-protected areas and being unable to tan on repeated sun exposure. The effect of inability to tan was stronger at younger than older ages. Lesser risks, with odds ratios of 2-3, were associated with being prone to burn on sun exposure, having many freckles as a child and having red hair. The site distribution of naevi in males compared with females resembled the distribution of melanoma by sex. Risk of melanoma of the back was significantly more closely related to back naevus count than naevus count for the remainder of the body. For other anatomical sites, naevus count was non-significantly more closely related to naevus counts at that site than counts over the remainder of the body. Naevus count declined with age in both cases and controls. In those aged under 40, having 100 or more naevi was associated with an aetiological fraction (AF) of 41%. In those aged 60 and over, however, the AF associated with this number of naevi was only 5%.