PCR assays were established for easy and fast analysis of two transforming growth factor-beta1 (TGF-beta1) gene mutations, a C to T transition at position 76 in exon 5 resulting in a change from threonine to isoleucine in position 263 (Thr263Ile) of the propeptide and a deletion of a C in the intron sequence eight bases prior to exon 5 (713-8delC). These mutations were evaluated in insulin-dependent diabetes mellitus (IDDM) patients (n = 137) and control subjects (n = 105) and in IDDM patients with (n = 170) and without (n = 99) nephropathy. After evaluating intra- and interindividual variation in TGF-beta1 expression levels, the TGF-beta1 mRNA level in phorbol 12-myristate-13-acetate-stimulated (1 ng/ml) lymphocytes from individuals with different TGF-beta1 genotypes was also studied. No association of the two TGF-beta1 sequence variations with IDDM in general was found. However, a weak but significant association of the Thr263Ile mutation with diabetic nephropathy was found (P = 0.03). No correlation between TGF-beta1 transcription level and genotype of any of the two studied polymorphisms was found. However, significant interindividual differences in TGF-beta1 mRNA levels were observed between the tested individuals (P < 0.0001) compatible with a genetic control mechanism of TGF-beta1 synthesis at the mRNA level.