The records of 46 patients with anti-(U1)snRNP antibodies and a minimal period of follow-up after first clinical presentation of at least 5 yr were examined with emphasis on symptoms contributing to established criteria of SLE, systemic sclerosis (SSc), RA or dermato- or polymyositis (DM/PM). At first clinical presentation 13 (28%) of the 46 patients studied fulfilled ARA-criteria for SLE (n = 10), RA (n = 2) and SSc (n = 1), and 33 (72%) were classified as mixed connective tissue disease (MCTD). During follow-up 18 patients initially classified as MCTD were now classified as SLE (n = 5), SSc (n = 7), RA (n = 3), or a combination of these disorders (n = 3). A transformation of MCTD towards these connective tissue diseases occurred 2.6 +/- 3 yr (mean +/- S.D.) after first clinical presentation. At the end of the follow-up period 67% of the patients fulfilled ARA criteria for SLE, SSc, RA or a combination of these diseases. The majority of patients with anti-(U1)snRNP antibodies have or will develop a classified connective tissue disease within 5 yr after clinical presentation. This undermines the concept of MCTD being a distinct clinical entity.