A retrospective case series of 53 female patients with incontinentia pigmenti (IP) including 28 secondary cases (female relatives of probands) was reviewed and compared with other series in an attempt to estimate more accurately the true disease burden of patients with IP. We found that, while the frequency of the first three cutaneous stages of IP was comparable with previous studies, none of the secondary cases manifested any serious neurological complications but all displayed stage IV pale anhidrotic reticulate lines on their posterior calves. This important clinical feature of IP in secondary cases has previously been under-represented in studies that often involved only paediatric probands. Hence, mildly affected cases of IP are often undiagnosed and under-represented in case series to date, possibly leading to inappropriately high estimates of neurological and eye involvement. With the availability of genetic testing, it is now feasible to confirm the variability of the phenotype and the risk of complications in IP.