Patients with idiopathic recurrent calcium nephrolithiasis (n = 57) and controls (n = 16) were investigated regarding the relationship between renal phosphate handling, other renal tubular functions and calcium metabolism. Incomplete renal tubular acidosis (RTA) was disclosed in 13 patients. RTA patients together with stone formers with normal renal acidification capacity (SF) exhibited low values for serum phosphate and renal threshold phosphate concentration (TmP/GFR) compared with controls. TmP/GFR was lower in RTA patients than in stone formers with normal renal acidification. Hypercalciuria of the absorptive type with normal serum PTH and urinary cAMP concentrations was a common finding in both stone patient groups, whereas no patient displayed unequivocal evidence of parathyroid hyperfunction. Fractional excretion of sodium was raised in both SF and RTA patients compared with controls. There was a positive relationship between the fractional excretion of phosphate and sodium in all subjects as a group. TmP/GFR was negatively correlated to fractional excretion of sodium. Twenty-three percent of RTA patients and 8% of SF displayed tubular proteinuria which often was associated with low TmP/GFR levels and enhanced natriuresis. It is concluded that a defective renal tubular phosphate handling is common in calcium stone formers and often associated with signs of other tubular dysfunctions. The altered phosphate handling seems to be unrelated to hypercalciuria.