PCR-based solid-phase minisequencing method was used to analyze the steady-state mRNA levels of the porphobilinogen deaminase gene in eight patients with acute intermittent porphyria. The patients had the earlier characterized mutations 517C --> T (R173W), 518G --> A (R173Q), 673C --> G (R225G), 673C --> T (R225X), 713T --> G (L278P), and 1073delA (frame shift). All mutations, except the missense mutation 517C --> T in exon 10, affected the steady-state transcript levels of the mutant allele. The mutant mRNA levels in lymphocytes varied from 5% to 95% of the corresponding wild-type allele levels. In contrast to the CRIM-negative mutation 517C --> T, the CRIM-positive mutation in the same codon 518G --> A resulted in reduction of the steady-state transcript level of the mutant allele to 65% of that of the normal allele. The two mutations, 673C --> G or T, affecting the same nucleotide in exon 12 also differed considerably in their effect on mRNA levels: The transcript level of the allele with a missense mutation was decreased to 80% of that of the normal allele, whereas a nonsense mutation at the same position resulted in a dramatic decrease (fivefold) in the levels of the mutant transcript. Our data showed large variations between the levels of mutant transcript in AIP patients and these variations did not correlate either to CRIM class, to the location of the disease causing mutation in the PBGD gene, or to the clinical phenotype of AIP.