Cyclosporin A (CsA) is a potent immunosuppressant that has many side effects, including hypertrichosis, gingival hyperplasia, and tremor. To evaluate whether there is a relationship between the CsA-pharmacokinetics (PK) and these side effects, their presence and intensity were observed in 46 renal transplanted children/adolescents during two regular visits, and the occurrence of the side effects was correlated with CsA-PK. CsA doses had been unchanged for at least 6 mo. CsA blood concentrations were measured at time 0, and 1, 2, and 4 h after the CsA morning dose. An abbreviated area under the curve (AUC) was calculated using C0, C2, and C4. Hypertrichosis positively correlated with C2, C4, Cmax, and AUC. An AUC > or = 4158 ng/ml per h was the best predictor for the presence of hypertrichosis. Tremor was also positively correlated with C2, Cmax, and AUC. A Cmax > or = 878 ng/ml was the best predictor for the appearance of tremor. These values of Cmax and AUC are within the therapeutic range of CsA as demonstrated by the studies of calcineurin inhibition by CsA. Gingival hyperplasia was not associated with any of the CsA-PK studied parameters. However, it was associated with the concomitant use of nifedipine. These data show that there is a correlation between the CsA side effects and its pharmacokinetics and that it is possible to decrease the incidence and intensity of such side effects by monitoring the CsA-PK parameters, providing they are under or at the proposed cutoff levels. Nifedipine should also be avoided to reduce the presence of gingival hyperplasia.