The possible association between lipoprotein(a) [Lp(a)] and albumin excretion rate (AER) is a topic that generates conflicting views. In addition, Lp(a) phenotypes have not previously been considered as factors influencing AER. In order to clarify this issue, we studied 70 non-insulin-dependent diabetes mellitus (NIDDM) patients without clinically detectable macroangiopathy, 27 with microalbuminuria and 43 without it. Both groups were matched for the known variables that could influence AER and serum Lp(a) levels. Lp(a) was determined by enzyme-linked immunosorbent assay (ELISA), and Lp(a) phenotypes were assessed by electrophoresis followed by immunoblotting. Lp(a) phenotypes were grouped as follows: 'small' (F, S1 and S2), 'big' (S3 and S4) and 'null'. The NIDDM patients with microalbuminuria presented higher serum Lp(a) concentrations than the patients without it [15.7 mg dL-1 (95% CI 0.5-36.5) vs. 4.5 mg dL-1 (95% CI 0.1-18.5); P < 0.001] and a direct correlation between Lp(a) and AER was observed (r = 0.34; P < 0.01). AER was significantly different when Lp(a) phenotypes were considered ['small': median 19 micrograms min-1 (range 1-195); 'big': median 9.5 micrograms min-1 (range 1-186); 'null': 4 micrograms min-1 (range 1-9); P = 0.04]. None of the NIDDM patients with a 'null' phenotype showed an AER of > 10 micrograms min-1. In conclusion, this case-control study provides evidence that microalbuminuria is associated with high serum Lp(a) in NIDDM without clinically detectable macroangiopathy. Furthermore, NIDDM patients with a 'null' phenotype could be considered at low risk for the development of microalbuminuria.