Psoriasis is associated with being overweight, but the temporal relationship is not known. This historical cohort study tested whether severe psoriasis resulting in hospitalization in adulthood was preceded by excess increase in age-adjusted body mass index, a known risk factor in childhood for being overweight in adulthood. The study cohort was based on the Copenhagen School Health Records Register, birth years 1930 to 1984 (309,152 schoolchildren). Cases were found through the Danish National Patient Register for the period 1977 to 2001. A total of 1074 (0.36%) of the schoolchildren were identified as having psoriasis, with at least one hospital admission. Multivariate analysis demonstrated an association between excess increase in body mass index and psoriasis in females only. Being overweight in adolescence was the main factor behind this observation. The female group showed a significant association between psoriasis and body mass index at ages 12 (p = 0.028) and 13 years (p = 0.010). This was not the case for males or for body mass index measured at ages 11 years and below.