['To measure MTX polyglutamates in circulating erythrocytes (E-MTX), mononuclear cells (MNC-MTX) and polymorphs (PMN-MTX) in rheumatoid arthritis (RA) patients and to see whether these correlated with clinical efficacy and side effects.', 'Sixty-five patients (40F, 25M; mean age 57 yrs.) with RA (ARA revised criteria) who had been on weekly pulse MTX (2.5-37.5 mg) for at least 2 months were entered into this study. The patients were classified as responders (R), partial responders (PR) or non-responders (NR) when blood was sampled for the MTX determination. Side effects since the initiation of MTX were also recorded. MTX-polyglutamates were measured (blinded to clinical details) using an enzymatic assay.', 'E-MTX in responders and partial responders were significantly higher (p < 0.001) than in non-responders. Similarly, PMN-MTX were also higher, but the difference was only significant for the R group (p = 0.0019). The differences in concentrations could not be explained on the basis of the dose, which tended to be higher in NR than in R (p = 0.085). The concommitant prednisolone dose was significantly lower in R than in NR (p = 0.001), as were the ESR and CRP (p = 0.007, and p = 0.05 respectively), but the MCV was higher (p = 0.047). E-MTX tended to be higher in patients with side effects, but this difference did not reach statistical significance (p = 0.15).'] Do methotrexate polyglutamate levels in circulating erythrocytes and polymorphs correlate with clinical efficacy in rheumatoid arthritis?