To evaluate the relationship between psoriasis disease severity, age at onset, and family history, we analyzed 537 US psoriatics, most of whom were from Michigan. Total body surface area involvement (%TBSA), presence or absence of joint complaints, and nail involvement were measured. Analysis of familial psoriatics revealed that %TBSA was 15.1% when onset was early, but only 8.7% when onset was late ( P=0.00003). The opposite trend was seen when psoriasis was sporadic: %TBSA was 14.3% when onset was early (</=40 years of age) compared to 28.0% when onset was late ( P=0.0034). However, the sporadic group was small and ascertainment of the sporadic group was biased for severe involvement. As determined by log-linear analysis, joint complaints and age at onset were not significantly associated after controlling for age at examination, nor were joint complaints and familial status. Psoriatic nail changes were conditionally independent of familial status, given age at onset; nail changes were more frequently encountered in early-onset patients. There was no significant difference in the frequency of carriage of the MHC psoriasis risk determinant in the familial vs sporadic groups. Early-onset psoriatics did carry this determinant significantly more frequently, as expected. These results demonstrate increased severity of skin and nail disease in early-onset psoriasis, when psoriasis is familial. The lack of clinical differences between "familial" and "sporadic" psoriasis may reflect a similar genetic basis for both conditions, at least when onset is early.